From what I remember, "new characters equals new jokes & dynamics to explore" was a main reason! I think one big element of adding Minrah was that we got to have an outsider's perspective on Belkar: the way she interacts with him is entirely different than how the rest of the team interacts with him, and helps to show how he's changed from original perceptions!
Side note: I've really enjoyed the character density of OotS. It never feels like there are too many characters or too many dangling subplots, and almost every detour has felt crucial to understanding and appreciating the main storyline. Just about the only part you could cut without losing too much context is the Bandana/Andi dynamic, although even that I really appreciated for a normie's insight into how crazy the OotS-verse has gotten. How do random everyday mercenaries react when they suddenly get caught up in a plot to save the world?
I've had this back-and-forth feeling about Haley & Elan's dynamic before: it's fun to have a fictional relationship where the guy is the complete ditz but still romantic, empathetic, and competent (in his own ways). However, some of the jokes of Haley managing Elan's behavior do read as very maternal, especially that exchange when he'd been lured away by Sunny which verged on feeling almost icky for me. I think their dynamic is at its strongest in scenes like bartering in Sandsedge, where they're both acting like adults but Elan simply has much lower savvy.
I disagree with that generalization. The vast majority of married couples I know treat each other with mutual respect.