I use NPC classes for characters who can't be defined strictly by adventuring classes (bartenders, wenches, craftsmen etc) or for NPCs who do have some amount of experience but were never adventurers per se.
For instance, a seasoned sergeant of the local Watch could be an 6th level Warrior. This man has experience. Maybe he has been on a battlefield sometime in his life. But he never actually went out there to find glory and fortune "the hard way". A middle-aged king who lived his whole life in a palace could be a high level aristocrat. An elderly woman versed somehow in the knowledge of magic, but who never wandered far off her community to discover the secrets of the universe could be a high level adept.
And so on.