I think you guys are missing the upside of all this: We'll finally be able to play as Garrus. Garrus! We'll be able to shoot sniper rifles and be badass and everything.
Honestly, it doesn't bother me. I prefer single-player, but sometimes co-op is fun. I'm not sure what the squadmates will do while Shepard is talking to someone, but that could be fixed by having co-op be a separate mode that doesn't contain the talky bits.
Going by what Casey Hudson is tweeting
I suspect "co-op MP missions" means just that. A separate, non-story mode where you play through in-game levels specially tweaked for co-op. I suppose it could be interesting if players each get to import their own personal Shepard into co-op...though I'm not sure if the logistics of that is anywhere near feasible.