Maxperson
Morkus from Orkus
I think the build the NPC like a PC rules were intended more for NPCs that adventure with the party. If I have 3 players and they need a 4th PHB built character to really shine, since the game is balanced around 4, I can make a Paladin or Cleric NPC to go with them if I want.The fact that we're not told what the CR of a character built by PC rules is does make it difficult to employ one as an antagonist, however. If I eyeball it, a 1st-level Fighter might be equal to a CR 1/4, but their AC would be quite high. Other classes get a bit stranger- like a 1st-level Wizard would be closer to a CR 1/8.
But things rapidly get out of hand. The CR 5 Gladiator has 100 hit points and 3 attacks per round, making them more like an 11th-level Fighter. So I think this way of building NPC's is best used for ones that are either allies or won't see combat.
My point, though, is that it is RAW for NPCs to be built like PCs, so NPCs as PC classes are a part of the fiction.