There will be no really significant consequence for using the tadpole and the game will end effectively if not exactly the same for those that do and do not. The "correct" way to play the game will be to, at a minimum, have a companion use the tadpole so everyone gets the super powers(bonus points if you use a companion you don't ultimately want endgame so if they are "lost" after Act 1 no big deal).
The shallowest game play all round will be a custom MC who never uses the tadpole(or forces a companion to do so).
Very well said. I think this would be my greatest fear for how BG3 turns out as well. But then again, the game has so many issues right now that it is practically impossible to pick out just one thing that would bitterly disappoint me.
Most everything else I might have issue with is mechanics, and I bought this game with the hope of modding the hell out of it anyway. I should be able to make all that as I like once the modding community has access to the full toolset.