It could just be like others said, after act 1, we do start running into more characters in later Acts. And those aren't Origin characters. Maybe there's only enough cure to go around for 4 individuals at the end of Act 1 to go around and that's the reason you lock your party (which I still think there's more to go, why would they abruptly end it at Moonrise Tower?) I think that's the true end, then after you get out of there and maybe? cure yourself, the bigger picture of the story will come in to play.

But anyway, what IF after the end of Act 1, we're locked into these companions, and I down the line don't like the route their story is going. Not as a player, but as my character. What if I don't agree with their ideas and want to dismiss them? Am I stuck with a 3 person party and a boring mercenary at that point? If so, yuck.

I realize this is ALL speculation and we could meet other folks after Act 1 and hopefully will. I realize they can't say certain things due to plot spoilers and whatnot. So why even make the mention of locking your party in the first place? All it does is lead to threads like this.