Yeah, it really is a big oversight.
If you play with NPC companions, you can dismiss them. If you play with recruit companions, you can dismiss them. But if you play with a character created by another player from the start, you're stuck with them for some reason.
With a game as long as this one, I feel that a lot of people are going to start their game with a couple of friends that won't stick with it till the end and get stuck with their characters.
Sure you can respec them with the mirror, but it would be nice to be able to kick these characters out.