I think resting and even exhaustion mechanics are pretty important and I like that both Pathfinder: Kingmaker and the first Pillars of Eternity tried to complicate this aspect.
The thing is, D&D, when played with strict adherence to the dice rolls, is very much a strategy/tactics games, at least in terms of combat. Its combat system was based off tabletop wargaming systems originally. A robust resting/exhaustion system adds a layer of strategic management to that, and it also has the convenience of fitting within the roleplaying aspect of the game. I think the more things the system has, generally the better. And I hope Larian puts some serious work into this aspect. The original BG system wasn't terrible for its time, but isn't good enough today.
Spells per rest and hit point pools and exhaustion can all contribute to a complex assemblage of factors that a player has to weigh when making their decisions. It's exactly what a CRPG should be shooting for. Moreover, it can also allow designers to create complex challenges in dungeons or wilderness areas, where parties are put to the test no only to overcome a single encounter, but to overcome encounters as efficiently as possible, because they may have to chain together several successful encounters to overcome that segment of the game. Instead of merely having challenges that are only one independent encounter at a time, which is something that's true of the OS games, for example.
