I think it's the same rationale as wanting psions to use a "different" system. A broad group of players want different mechanics to delineate different fictional spaces; I think it correlates with having broadly simulationist desires.
It definitely does not correlate, at least not reliably, to any sort of simulationist desires. Not how most people use the term, at least, which relates to mechanics representing the physics of the world.
Meaningful differentiation of character facing mechanics along the lines of fictional themes is about, well, exactly what it sounds like.
I just don't see any reason that having a martial character whose martial abilities are powered by spell slots creates any issue in that context, because what the character is doing is still very mechanically distinct from casting spells. What is important is that
what the character is doing, ie their actual actions in game and mechanically, are meaningfully distinct from abilities with a very different thematic flavour.
Basically in a game with distinct, prescribed, rules widgets that can be used selectively, it's a waste of design space, IMO, to
not make those widgets thematically and mechanically distinct. If playing a paladin feels the same as playing a fighter, there is no point in having classes and distinct ability sets. Just give characters combat skills, and use a broadly defined set of types of actions a character can take, and let them decide how to flavor a given action, at that point.