Astarion is power-hungry, and feeding that ambition makes for a rich story arc. Arguably he's more fun in an evil playthrough, but he still has a redemption story arc as well. And that story arc works well, IMHO, or well enough to make him a welcome part of a "good" adventuring party.
My point is that there's no in-story reason for Minthara to belong in a good party, and adding that character arc would be a lot of work for Larian. They wouldn't or shouldn't have to change her personality for an evil playthrough, but she would have to change in-character as part of a good playthough. And perhaps that's a bridge too far.
