I fully support the idea of taking Minthara prisoner on a Good run, but she should be able to get out of the cage and become a companion (optional)
I doubt Minthara would have any interest in joining a group that imprisoned her. She's a Lolth-sworn Drow, and from what we've seen she's an extremely proud personality, and fierce warrior. She would probably rather die in the name of the Absolute like a true zealot than join the campaign at that point.
I will say it might be fun to have her try and persuade us with her illithid power to set her free or join her.
She left Lolth for Absolute because she didn't like Lolth's cruelty and betrayal.
Although, the Absolute makes her do the same. If the Absolute is Lolth, who plays with tadpoles, then Minthara does not know anything about it
Minthara definitely doesn't want to die when she is defeated, and she has no choice because the Absolute will punish for failure and Lolth will punish for defection.
We have the evil Gityanki Laezel in a good party, why not add an evil drow?
She is not as evil as she seems at first glance, and is capable of being quite cute under certain circumstances (like Laezel). Craving for domination and power, pride and arrogance - this is not bad if we are on the same team and no one forces her to betray us, she herself will not do it