Not usually for M&M classes, I think its because adults and children compete on equal terms and children can't wear spurs for showing (correct me if I'm wrong anyone). You may gat away with it at local level though...
At local level there are usually no rules against wearing spurs in any class (including m&ms). Some judges may dislike it, but if your pony goes much better with spurs, it may be worth the risk.
If you're doing affiliated classes, you can't wear spurs.