I don't really see one, as long as you can still change after you've picked your champ. One thing I like about blind is that I can call mid, pick karma, then see that we have a wombo combo team and switch to orianna.
So I guess the downside is time spent on developing and bugfixing the feature, and getting people to accept it, when that work might be better spent elsewhere. I mean, blind isn't that bad.
I think going off of that, it'd be great if playing in Team Builder you could just pick a role and a tentative champ. Once you get into the team you could then readjust and switch champions if you'd like. Now to keep with the idea of the captain building their own team whenever someone wants to switch they have to request permission first so the captain can veto it just like they can kick people wanting to join. Hey, maybe even if you get in a lobby and like the teamcomp/people then you could request to change roles (say if you decide you really want to support that nice lucian, or see that you could play a mid champ that would make the team-comp. Just leave it as a request system and that would give the game-mode more flexibility like blind pick does