"On occasion", where appropriate.
Commoners, all the time. They're the fill of average, day-to-day people who live and get by in the world.
Experts, all the time. They're they broad range of people who are skilled in whatever field but certainly don't fit the mould of being a bard or a rogue. There are plenty.
Aristocrats, absolutly. The overwhelming majority of upper class folk are either exclusivly arisocrats, or have levels in both arisocrat and some other class. Brash nobleman who has a rather nasty repuation for his skill in a duel, ala Thibault from Romeo & Juliet, would be an Aristocrat/Fighter, Aristocrat/Swashbuckler, or similar.
Adepts, yes indeed. Though I'll admit I do use a house rule that states an adept has a fully customizable spell list in accordance with their concept. Adepts fill the roles of those various pseudo-magical people who wouldn't feel right as a member of another class. The wisewoman healer, the tailor who's the latest scion of his family's secret of making magical clothes, the witch who lives in the bog.
Warriors... eeh, occasionally. The rather weak distinction between fighter and fighter-that-sucks seems rather frail and artificial to me, so the class rarely sees the light of day. For most fighting men a small number of levels in fighter seems to make more sense than more levels in warrior. The other NPC classes do a good enough job justifying their own existance, the warrior does not fare so well.