I said "on occasion," but I should clarify.
I don't stat up the vast majority of my NPCs. The halfling who runs the gladiatorial arena? The merchant rival of the PC sorcerer's father? The military governor of the occupying armies? None of these are people the PCs are going to engage in one-on-one combat. I have a sense of what level their important skills are, if I need to make an opposed Sense Motive check or the like, but I see no reason to take the time to stat them up at all.
Of those NPCs who do get stats, most use adventuring classes, because they're important characters and/or intended for combat. I use NPC classes rarely, but I do use them.