I live in central India and I haven't met many people who only worship one particular deity or God. They may be partial to one over others, but they worship all dieties. Most temples also contain multiple idols.
I wonder if there are any benefits to this path of God realisation over worship of one single deity.
The answer to this question becomes more of an open-ended opinion-based discussion rather than one with a definitive & authoritative answer . So brace up for a long answer.:))
As it is with the Sanātana Dharma aka Hinduism, there's no singular monolithic directive authority, since it being more of a loose conglomerate of various Schools of Thought and Sects (saṃpradāya). Thus the view and perspectives on whom or what to worship may change with place, time, the person you have considered as your "guide" and/or guru, etc.
The oldest of Vedic Texts aka the Rigveda says :-
Ekam sat viprā bahudhā vadanti
एकं सद्विप्रा बहुधा वदन्ति ।
i.e. The Truth is one, the wise express it in numerous ways. [Rig Veda Samhita, 1.164.46].
There's a famous Subhāshitam (सुभाषितम् - a sanskrit aphorism or maxim), which can also be found in several concluding parts of phal śrutis (फल श्रुति ) in Shri Vishnu Sahashranamam Stotram (श्रीविष्णुसहस्रनामस्तोत्रम् ).
ākāśāt patitaṃ toyaṃ yathā gacchati sāgaram | sarvadevanamaskāraḥ keśavaṃ prati gacchati ||
आकाशात् पतितं तोयं यथा गच्छति सागरम् ।
सर्वदेवनमस्कारः केशवं प्रति गच्छति ॥
Meaning:- As all the raindrops falling from the sky ultimately meet their end in the ocean, the worship of any divine God ultimately reach the one Supreme Lord.
In Srimad Bhagvad Geeta [BG. 7.21], the God says:-
yo yo yāḿ yāḿ tanuḿ bhaktaḥ śraddhayārcitum icchati ।
tasya tasyācalāḿ śraddhāḿ tām eva vidadhāmy aham ॥
यो यो यां यां तनुं भक्तः श्रद्धयार्चितुमिच्छति।
तस्य तस्याचलां श्रद्धां तामेव विदधाम्यहम्।।
Meaning: Whatsoever form any devotee desires to worship with faith that (same) faith of his I make firm and unflinching.
Thus, in a way you’re basically worshipping the same “deity” in different forms. (Obviously, there can be several other interpretations to all of the above verses leading to different perspectives and interpretations which itself innately leads to the heterogeneity within Hinduism ).
The parable of Blind men and an elephant, explains the above viewpoint of everything there is, as a manifestation of one Ultimate truth, quite effortlessly.
Now question is
I wonder if there are any benefits to this path of God realisation over worship of one single deity.
As interpreted above, since ultimately it's the same Supreme entity you're worshipping (through whatsoever method or to whosoever be it) So, does it matter at the end which path was taken when the destination to be reached is indeed same.
Another interesting interpretation regarding the Significance of worshipping multiple deities which I have always heard from my elders and watched (in some form or another) is the "Analogy of a Nation".
If the Supreme Brahman - परब्रह्म, [ which might or might not get a Specific deity name (with some attributes or not - saguṇa or nirguṇa ) such as ādi Shakti or Para Shiva or Vishṇu/Nārayaṇa/Para Vāsudeva, etc - depending on the fundamental thelogical and philosophical views of the concerned sect ], is to be considered as the Head of the Nation then consider all the other deities (as per that particular sects' sectarian defined hierarchies) as working under the command of that Supreme Brahman - S.B. (who can be thought of as the Prime Minister in the Indian context)
As the PM distributes various portfolios to his different ministers, so does our S.B. , who have distributed all various departments of the universal existence to different Gods/ Demigods/ Deities, etc. So, like if you want your Driving License to be made, you won't go the Prime Minister directly rather approach the local government administration officer. Thus, if we want a particular desire of ours to be fulfilled we must pray to the deity in charge of. For example: Remove Obstacles = Lord Gaṇesha, Wealth & Prosperity = Goddess Lakṣmī, Knowledge and Arts = Goddess Sarasvatī, etc.
On the contrary if you have direct connection with the P.M. chances of you being in an ever blissful and happy state, increases manifold and thus no requirement for pleasing any administrative officials.
I have always found this analogy silly in some form or other, but in some strange ways it seems to work (at-least for me).
Moreover, generally the Supreme God of any of the sects in hinduism are always the final goal to be attained or learned by the beleiver. So if that's your aim, then only one Supreme God need be worshipped perhaps.
Of the Four Puruṣārtha (पुरुषार्थ) - Dharma, Artha, Kama and Moksha (liberation, spiritual values), Moshka is generally considered as the final goal of our life to be attained (as per all school or sects), which in general, can only be provided by the Supreme Deity of the particular sect. In that way, it seems more logical that you worship only the Supreme God (S.B.), in whatever form you might like or whatever school of thought or sects' ideas influence you the most.
Generally, the Smarta Tradition - (स्मार्त) , whose founder is generally attributed as the Adi Guru Shankaracharya, is considered as the blend of all the core philosophies of Hinduism and advocates for multiple deities worship in some form of other along with ones' own personal tutelary deity [इष्टदेवता, iṣṭa-deva] .
So, to conclude, depending on ones' material and/or spiritual desires and affinities one may choose their deities for worshipping.
As per the advice that most Gurus give, it's not beneficial to worship multiple deities for any particular individual. One must only worship his/her Ishta Devata and stick with it's worship only (without of course having any ill feelings towards other deities). Temples have multiple deities for other reasons but even then the presiding deity (or the Pradhana Devata) of a particular temple is always a particular deity.
My first Guruji used to say this always in his Satsangs. He cited examples of some famous saints of Bengal who always had shown one-pointed devotion towards only one particular deity. For example, Ramakrishna Paramhamsa always used to remain devoted to Goddess Kali and her only, Sadhak Bamakhepa used to worship Goddess Tara and only Tara. Similarly, Trailanga Swami used to worship Lord Shiva and only him whereas Chaitanya Mahaprabhu was devoted only to Lord Krishna. Guruji used to say that all spiritual aspirants must follow these examples and stick to only one deity in their particular lives.
As for some other reference, for the time being I am able to find a reference from Sri Chinmoy:
Stick to one god
There were two friends who were very spiritual. One of them eventually became a great seeker, but for some time he used to do something that seemed to indicate that he did not have any real depth: every day he would worship a different cosmic god or goddess. This seeker became very well known, and many people would ask him to pray to the cosmic gods or goddesses on their behalf. He did it quite happily. He used to tell them, “If you like Lord Krishna, then I will pray to Lord Krishna. If you like Shiva, then I will pray in front of Shiva’s image. If you like Mother Kali, then I will worship Mother Kali. It is up to you. Most sincerely I will pray on your behalf. But I cannot guarantee the results.”
So, people would give him money and then ask him to pray on their behalf for the deity of their choice to bless them and solve all their problems.
The seeker’s friend heard that he was making lots of money by worshipping this god and that god. This friend was not at all well known, although he was also an excellent seeker. He realised that his dear friend was doing something which would seriously affect his own spiritual progress. So one day he came and dug five deep holes in front of his friend’s house. The great seeker came out and said, “What are you doing? Why are you digging so many holes? Is one not enough for you?”
His friend stopped digging for a moment and said, “I am making these holes with the hope that, from one of them, water will spring up.”
The seeker said, “What do you mean? How will you know that any hole has water if you do not dig it deeper than all the others? My friend, I advise you to concentrate on only one hole!”
The friend smiled and said, “You have to give me this piece of advice? Every day you are praying to a different cosmic god or goddess with the hope that one day one of them will be pleased with you. By constantly changing, you are not giving the proper amount of time to any one of them. That is why I have dug these holes in your garden. It is high time for you to realise that if you want results, you have to stick to one god or goddess.”
This was how the friend taught the great seeker a most significant spiritual lesson.
NOTE: I am doubtful about the validity of my source here. Although, Sri Chnmoy is a spiritual teacher, I doubt whether his opinions will be considered by the others as authoritative or not.
The purpose of worshiping multiple Gods is because of multiple desires fulfillment. For example worship of Indra is done in order to please him for rain. Worship of Kuber is done for the wealth etc. But one must stick to Supreme God or Parmatma in order to God realization.
Swami Muktananda commentary on Bhagwat Gita Chapter 9 verse 25 clarifies this doubt:
Devotees can only be elevated to the level of the entity they worship, just as water in a pipe can only rise to the level of the reservoir to which it is connected. In this verse, Shree Krishna explains the implications of worshipping different entities by revealing the varieties of destinations attained. He gives this knowledge to help us conclude that to reach the highest level of spiritual evolution we must worship the Supreme.
The worshippers of Indra (the rain god), Surya (the sun god), Kuber (the god of wealth), Agni (the god of fire), etc. go to the celestial abodes. Then, when their account of good karmas gets depleted, they are sent back from heaven. The Pitars are the ancestors. It is good to harbor thoughts of gratefulness toward them, but undue concern with their welfare is detrimental. Those who engage in the ancestor worship go to the abodes of their ancestors after death.
The highest devotees are those who attach their minds to the Supreme Divine Personality. The word vrata means resolve and undertaking. Such fortunate souls, who firmly resolve to worship God and engage steadfastly in His devotion, go to His divine Abode after birth.
