Yeah no selfish motive ...
Except potentialy biger reward (if your dice roll sucess) ... potentialy more powerfull alies (goblin army vs. few dozens of poor refugees) ... option o infiltrate (or join, depend on your future plans) cult with guaranted major role (since all true souls are in leadership) ... and quite obvious conection to Goddess, whos magic (even if a bit darker) seem to allready saving your life from that ugly toothy tentacled slug in your head.
No motive at all ... sure.
How much of this is offered to you, and how much of it did you come up with as personal justification? I don't say that to deny the truth of those things, I just think that more of this should be offered up front. It wouldn't be hard to have Minthara promise these allies, answers, connections, etc. in her dialogue. She could even try to convince the main character that the tadpole is a good thing that gave them great power (Astarion would agree, he's very pro-exploit the parasite power).
But yeah, I guess what I mean by no motivation is that none if offered to the player. It's either a chance at great power and allies or a more guaranteed chance at removing the parasite.
