The other points are: campaign building and story telling. Like it or not: but Larian seems to want to give every companion a unique story and interactions. In short: make them really great characters and bring a story for all of them to life. If you wanted to have a max party size where you still have relevant story and things like members leaving because of the descisions then you potentially have to make just true neutral characters which are kinda bland, or you have to do more origins to account for potential loss, while still maintaining the 6 PC party size. Seems like reasonable reasoning to me. Look at the older games, which had more companions: none of them were half as developed as they are today (which is fine, but its still a huge difference).
I don't know, man. I'm pretty sure I'd take Nalia or Jan over Astarion. Probably even Yoshi too, up to the point where I get Imoen back. I'm pretty sure that I'd pick Mazzy over Laezel and I'd probably pick Minsc too, but I guess I'm too soft-hearted to not send Keldorn back to his family while he still has one. And if you really want character then how about Morte, a literal bonehead with a zombie-fetish? Don't tell me you didn't like Dakkon either, even if he was a rather brooding type. I can still imagine him saying "endure! In enduring grow strong" in his best Director Skinner voice. Annah was pretty cool too, for a stereotypical teethling thief. Just remember, don't touch her leather.
Do you think the memory of Gale or Wyll or Laezel will stick around for 20 or so years?