Oh it definitely adds replayability for sure. I always enjoy that in a game. But if it's corralled into one area of the game, that's not all that fun to me. Imagine playing through Act 1 repeatedly just to get to Act 2 with your different party members. Granted you could just make a separate save right before the end of Act 1.
At the start of ToB, you can summon all your various allies at a camp of sorts. At the end of ToB, you still find out what happened to all your members. It was a nice little bookend. If I never see my companions again after act 1, it would be disappointing. Again, that's a lot of speculation on my end, who knows where things will go.
