I really, really don't like this and I've commented on it a couple of times already. Limitations to party should come from story reasons (like disagreements etc.), not handwaved "rocks fall, everyone dies" or the like. There are so many problems to this approach.
True, but it does solve one big problem in game design. It can eliminate running across quest triggers for companions you don't have in your party. They just don't exist, so you don't get into that mode where "Oh, I have to trade this one out for a while and pick up that other one to do this side quest."
That also means you're not seeing all the content in the game in one playthrough, but personally I don't mind. It's like the branching storyline in Witcher 2; you have to commit to one branch and will only see the alternate reality with a second play-through. I'd rather have this, than run across side quests I can't do because I don't have the right companions in my party.
