Yeah I don't mind party members splitting up when it doesn't seem like they're doing just to do it or keep you from having to switch party members early or at all. I do feel like they did that to Quina in IX though. The first time they left her at Madain Sari and Zidane responded that "he/she'll be fine on his own". I feel that was a clue that Zidane doesn't like Quina at all and regretted allowing her to come along and at the same time I felt like the developers were just doing it to prevent us from having to switch party members before they wanted us to.

No actually, the first time was at Cleyra when Quina decided not to jump into one of the orbs because "I no like heights". I felt like that was just another cop out and Quina surviving that seemed unrealistic considering that Brahne destroyed it like a minute after that scene. There was no way Quina could've made it to safe distance in that time.