I can see a reason for her to be even in party of good or neutral PC (when we join tieflings ofc, so before that slaughter, which makes her have way less blood on her hands), but this would require the Larian to implement the knockout mechanic and possiblity of capture.
There already is a knockout mechanic, it's just not working quite properly yet.
I have written this badly here, sorry. I meant exactly what you have said; same thing was written by me in other thread, linked above.
Knocked out characters bug the quests, aren't registered in them or just get slaughtered by nearby npcs. There is little interaction with them afterwards.