This game will probably have more possible dialogue impacts and deviations than any other game they've made, and most definitely more so than the original 2 BG games. I get the limitation of locking your party down based on that alone, it would be a nightmare to have to account for all the various deviations between an interchangable party. Sometimes awesome things like that require sacrifices. On top of that, I can't imagine how companion quests would work if you could so easily miss something because you didn't have X party member in your party during an encounter.
Honestly it's a little funny to see people shout "DEALBREAKER" every time some information comes out that goes against their expectations. People have such a rigid view of what this game should be, it seems like there isn't any hope of people simply giving Larian's vision a chance. Design decisions like this are made for a reason most of the time, and it's at least worth considering what those reasons might be and that just maybe it might be the best thing for the game.
