I agree, co-op doesn't work very well in a game built so heavily on character development/relationships and dialogue.
Dialogue choice actually makes up a pretty small portion of mandatory gameplay in the games. Obviously if you were wandering around looking at all the npcs in between missions, that starts to increase time spent, but why is anybody assuming that people running the game coop would be doing this?
On the flip side, if they chose to do this, then why is it an issue?
Everyone keeps acting like their friends are going to hold them hostage for 2 hours in between missions. This just doesn't seem like a logical argument to me.