I think you're forgetting how flat many of the characters are in BG 2 with maybe 2 -5 lines of voiced dialogue each and how much work Larian has left to do. BG 2 could only rope you in to caring about a companion if they had their own quest or lots of backstory(all delivered through text.) Now every companion has to be like that and it means we get less companions but more complex backstories for them. I believe less is more and wanting to have support for a larger party is fine but it is not a priority for them right now and that is reasonable.
BTW Dragon Age Origins set the precedent of modern 3d RPGs using 4 party members (devs called it a spiritual successor to BG) and it was quite effective and fun.
