For me, Minsc is too big a character to be playable. He has an established personality and no, I can't see myself playing him as evil. Maybe as a weird one-off, but not for the whole game.
I reckon he'd work best as a quest-giver or someone who the party can turn to for information. Maybe a quest to find Boo.
I agree, for me he would probably never pick him as an MC beyond curiosity, but if he is able to be recruited I will be extremely disappointed if I cant become his "witch" when playing as a mage type character.