On the other hand it'd be really annoying to keep his pre-recruitment stats. I can't remember them of the top of my head but I believe it was something like 3 int and 6 wis. From a role-playing perspective I agree with OP, but looking at it from a gameplay perspective he'd be a major liability if he kept his pre-companion
stats.
On a side-note, I always used Minsc in BG2 and he was always killed by Mindflayer INT drain and charmed by anyone and their mother
(obviously building around it to reduce the risk, keeping him raging etc. but that illustrates the gameplay issues you'd experience with more role play-friendly stat allocation)
A similar thing/issue would be Jaheira. She's companion level when you first see her in action but is heavily nerfed in the Moonrise fight. At least if she joins your team and becomes your "minion". It sure breaks the immersion a bit, but I understand it form a balance perspective.
