Personally, I get additional value out of building NPCs consistent with the PC rules (even if not to the same level of granularity) because I run sandbox and semi-sandbox games where encounters are not designed to be level-appropriate. Accordingly, I need to be able to telegraph the threat posed by likely foes (either on my own initiative or in response to the PCs trying to investigate a potential enemy), and it's a LOT easier to accurately telegraph threat levels when the NPC is built according to a framework the players are familiar with. For example, if I show a human swordsman making three attacks with one action, my players are able to infer that the NPC has at least eleven levels of the Fighter class. If I didn't build NPCs according to the PC rules, then showing the enemy making 3 attacks doesn't permit any inferences about the NPCs other abilities.
As a bonus, when I do give an NPC an ability or combination of abilities not easily explained by class levels, my players are able to infer that something special is happening, and that it might be worth investigating (whether to co-opt, disable, or otherwise thwart). If instead any NPCs could have any collection of abilities, I'd have to explicitly highlight anything meant to be special, which my players and I find less fun than letting the players discover the intriguing inconsistencies on their own.
As a player, while I can and do happily play at tables without PC/NPC symmetry, it does drive me bonkers when an NPC has a monster ability thematically similar to my character's abilities, but implemented with incompatible mechanics. For example, I once played a Thief Rogue who spent lots of money buying the advanced poisons from the DMG, and dutifully using Fast Hands to apply those poisons to my weapons. It was maddening to fight against Drow who had monster abilities that let them deal massive poison damage with every attack without permitting saving throws, without needing to repoison their weapons, who weren't constrained by limited supplies, and whose leftover poison couldn't be looted.