I never played MotB but I played the BG series and in it resisting the pull of evil came with mechanical benefits - lesser benefits to be sure but benefits nonetheless.
I wish there'd be a way to get rid of the Emperor earlier, release Orpheus, side with him and get some mechanical bonus from him in return.
That said, for me the mechanics wasn't the motivation to use the tadpoles. But when Lae'zel
saw Orpheus sacrificing himself to become a mindflayer
, I couldn't keep her from following his lead and devouring the whole pile of tadpoles within a second.