For a single player experience I think it is a bad design - having companions be “player’s equal” creates narrative issues. And while Bioware’s “chosen one the leader” trope got pretty stale throughout the years, so far Larian’s approach with vague protagonist didn’t work for me so far (mainly D:OS2. I am judging BG3 via gameplay videos due to my hardware limitations).
For coop though, it’s brilliant. Companions being playable means coop players can drop in&out.
Why does having multiple important stories integrated together crate narrative issues? Is it because we are programmed to love and follow a single hero and there has to be a solitary leader figure that pulls everyone forward through the story? Why can't there be a team of equally important and different characters and the team as a whole is the "hero" that pulls the world through the problems and saves the day. I am loath to use this example because I think superheroes are played out....but who is the hero in the avengers? Is it Captain America? Iron Man? Thor? Dr Strange? Antman? Blackwidow? Hulk? Nick Fury? (not Hawkeye though..... :p )
This became a plot point and a struggle in itself, as the team dynamics had conflicts on what they felt was important. Hell even the Teenage Mutant Ninja Turtles had conflicts on who is the leader, but they heavy handedly pointed at Leonardo for the most part.
So in this game/story there are all equally important characters that can influence the story in various ways, and while you are the defacto "protagonist" that drives the story forward you are not necessarily the most important one or the hero, or the special. This is an acceptable story arc and has been used as the "bystander" to greatness before, but we are more than a bystander,
Yet we don't have to be the "lone avenger of super heroic manliness second coming of what ever deity figure destined to save the world and be revered by all and infinitely charismatic unifier of badassery"...yet it seems that is what people want to see in stories...which I feel is somewhat shallow (sorry).
Now are the companion stories compelling enough or just hamfistedly put in the game and ultimately meaningless? Maybe, we will find out. Some may be more interesting than others.