This depends heavily on playstyle. At some tables, including mine, enemy spellcasters rarely start with all their spell slots available unless they're encountered immediately after they took a long rest. Even at tables where that is not the default, I'd like to think that if players deliberately engineer a situation where the NPC has a reason to cast a bunch of spells prior to the combat encounter with the PCs, that the DM will honor the players' agency and have those spell slots remain expended when the fight begins.
As written, because the new-style statblocks have uses per day for each spell rather than spell slots, strategically attacking a spellcaster when they are low on slots isn't feasible mechanically (you'd have to deplete their uses per day spell by spell, and they'd still have a massively powerful primary attack usable at-will). As a DM, I either need to not use new-style statblocks or else rewrite my campaign setting so that the inhabitants understand IC that the primary strategy for dealing with spellcasters--attack them when they're low on slots--isn't reliable any more. Sure, I could rewrite the new-style stat blocks to rely on depletable spell slots, but then I wouldn't be using the new stat blocks, would I?
So while I appreciate that some DMs preventing PCs from using attrition tactics (or just attacking late in the day) by always running NPC spellcasters with full slots may have been a problem, the new-style statblocks "fix" that problem by preventing any PCs from using attrition tactics against them. I have a hard time seeing that as "pro" of the new statblocks.
(To reiterate, I understand that the new-style stat blocks are not unprecedented in 5e. But their increasing usage in newer books makes the problem described above more acute.)
