Worshiping Demigods is not bad at all. Demigods worship is however condemned in Bhagavad Geeta, It is not Forbidden.
Here in Gita Krishna says that Demigod worship is inferior to His worship.
Those whose intelligence has been stolen by material desires surrender unto demigods and follow the particular rules and regulations of worship according to their own natures.
Men of small intelligence worship the demigods, and their fruits are limited and temporary. Those who worship the demigods go to the planets of the demigods, but My devotees ultimately reach My supreme planet.
Demigods are the celestial beings with some super natural powers. They are more powerful than human beings.However they can grant only material benefits. That is some comfort for the body; ex. Indra can give nice plentyful of rains, Vayu can give good strength to man etc. But they can not grant liberation to living entities(soul). In Shiv puranaAs Shiva says(As quoted by Swami Prabhupada) 'Mukti Pradata sarvesham Vishnu eva na samshaiyaha' Only Lord Vishnu can grant liberation to soul.Of this there is no doubt.
Lord Vishnu is the supreme personality of Godhead(Vaishnavas believe this, Shaivas believe that Shiva is supreme). Vishnu being Absolute truth; He is non-different from His other incarnations and expansions. So all other celestial beings are demigods; but not Vishnu and His expansions(Since I'm Vaishnava I call Him supreme). So yes Ganapati, Kartikeya etc are demigods.
The reason why demigod worship is condemned is that; Bhagavad Geeta speaks about the highest perfection(liberation; which demigods cant give anyways). The Goal of Human life is to attain liberation from cycle of births and deaths. That this is the ultimate Goal of Human life is accepted by all the followers of Vedanta philosophy.
To give a crude ex. consider a university. In University there are different department Heads; also there are professors, clerks etc. So Departmental heads have some power which is more than that of professors, Power of clerks is still less than Professors. But principal is the authority in college. There is no one higher than him. similarly there is 1 supreme person(Krishna/Vishnu). Demigods are like HODs. They have some power but it is given by principal(supreme being). Principal can override any one's power if he wills . So its better to worship supreme than to worship His subordinates.