I also agree that there should be companion options that aren't part of the core "playable" options further along in the story. Even if they are slightly less engaging. It is fun sometimes to find that warrior whose only response is in grunts most of the time, that you can manage to get a few key lines of dialogue out of once and awhile, or a monk that has taken a vow of silence... though I guess that becomes less true if you are able to read their thoughts later... hmm...
If there is a good story reason for it like the companions you don't choose turn into mind flayers and become major villains in the story later I'm okay with the idea of having to pick which companions you are able to save. I was attacked in the night by Astarion and decided he was annoying and more trouble than he was worth anyway and decided to kill him. If it is just an arbitrary thing to cut down on how much of the story you can experience in one playthrough though I'm kind of against that.
I also wouldn't mind if someone hopped into the party occasionally as a companion type creature for a zone or something as long as we were given the option to take them or not. Just no escort quests
