I think one good argument for locking companions after Act 1, is that the game can be programmed to avoid giving you triggers for side quests requiring a companion who isn't in your current party.
I'm not a fan of finding quests in a game that strongly hint, or even require, that a certain companion has to be in your party to complete the quest. I don't want to be railroaded into my party makeup, even temporarily to complete one quest. If the party is locked after Act 1, the game can just not show those other companion side quests.
Of course a counter-argument is if you enjoy that sort of thing, and you want to see every possible side quest in a game rather than re-play with different companion selections. Even if it means frequently re-arranging your companion choices.
As a side note, I'm getting the horrible feeling that the way Act 1 will be companion-locked, is that the ones you don't choose will all turn into Mind Flayers. Or at least proto-Mind Flayers in the process of changing. And then you have to kill them in the camp before moving to Act 2. This is heavily foreshadowed in the camp dialog about how you'd prefer to be killed if you start changing, Laz'el vowing to kill everyone and then kill herself last, and so on. Maybe that won't happen and they'll all just wander off, but it would certainly fit the theme the game is developing.
You are raising potential issues that gemes from 2004 dealt with. Ever since Knights of the old Republic, Bioware had a very specific way of doing companions side quests. If Larian really don't know how to handle this problem, why not look into these games for solutions? Is there a list of game mechanics Larian is allowed to copy from Bioware and a lost of mechanics they can't?
I don't necessarily say that Bioware did it best, but I think taking inspiration from them is better than Larian "solution" (which is plain lazy and borderline stupid)
