If NPCs are built by consistent rules for themselves, even though those consistent rules are not the same as the consistent rules for PCs, would that not give both benefits?
Again, this is a bit pointless.
It only works if your playing a very limited game with a small rule book. Then you can but that small rulebook on a pedestal in the middle of the table and say "all NPCs are made as per the rules on page 11". And that is great for some games.
Other games though, not only have more rules and content, but also the DM can make stuff up on a whim. There are no time traveling Were elephants in the "offical rules", but a DM can still add them and give them a "time travel trunk" special ability.
It's pointless to stat out a lot of NPCs. Like the DM just needs a Seer NPC to cast "legend lore''. The DM can just say the NPC has that ability....or, if the players demand it, make the NPC a 12th level cleric. Ok....so the players do a happy dance as they forced the DM tp make the NPC a 12th level cleric. But it does not mater as all that NPC will ever do in the game is cast legend lore once.
It feels like something that would very rarely come up in actual games.
It does depend on the game.
Though we are really only talking about two types of NPCs: The Story NPCs and the Combat NPCs.
A story NPC has an ability as part of the story. The seer can see the future. The guard can see invisibility. The elven princess can polymorh into any animal. Some thing like that.
And the bad, exploitive player will say "I demand my character can polymorph at will into any animal just like the NPC elf princess!" as part of their exploit to ruin the game.
And the Combat NPC just has an ability for combat. The Fire Goblins can each shoot out balls of fire at will. And again, the bad, exploitive player will want to use and exploit that.