My personal theory is that Larian decided on including Jaheira and Minsc very late in their development process. They originally intended to have two or three completely new "good" companions in the game. But then they got sidetracked with all manner of other things they were excited about working on, and completely dropped the ball on creating those new "good" companions. So then, at a very late stage, they decided to (re)use Jaheira and Minsc as a way of quickly bringing in two "good" companions without having to do a lot of work on them. This is what I feel happened.
From the dataminig at the start of EA it seems like Minsc was originally planned to be an origin character and then was later changed to a nonorigin companion (rather than not planned at all).
I would be happy if there were several other nonorigin companions in addition to Minsc and Jaheira!🐻
