Two Sets of Rules, the Good and Bad?

[Tanarii]

I've seen a number of thread where folks have been told, in effect or fairly explicitly, that if they are doing NPCs as PC builds they are Doing it Wrong^TM

Of course, the most common time this comes up is someone trying to figure out how level maps to CR.
(Short version is it doesn't, you need to compare it to the DMG table and see what CR comes out.)