I wouldn't want companions spaced out between the acts, personally. I don't see what the benefit would be.
If some companions only become available late(r) in the game, odds are, by the time you get to them, you won't need them. You've already worked out your party composition, decided who you liked personally, maybe even got invested in their storylines – a newcomer doesn't stand a chance. They're more likely to get benched and ignored for the rest of the game, save for their personal quest.
Plus, having less options for constructing your party from the beginning, you may feel pressured to play a certain way or a certain class to plug holes. It's a problem BG3 already has, and spacing out companions will only make it worse.
A huge +1 to all of this. Agree completely. I HATE getting party companions late in a game and never ever use them other than to complete any quests specifically associated with them.
