Tav Nobody fan here -- it could be that everyone else in the party is actually special, carefully handpicked, etc. - but Tav was just the test subject for the insertion of this very special parasite, and was supposed to be totally disposable. This would put her on the same boat as Karlach, who was probably infected not by a grown up doctor tentacle, but by a rogue tadpole crawling into her eye after the Nautiloid crashed.
Anyway, having "special chosen ones" and "normals" who are able to rise to the occasion would make the character dynamics much more fun. "Yeah yeah Gale you are the Chosen One Twice Over, I still get the same unholy tadpole powers so chill please".
