I clay and polish twice a year, you have to clay, the reason you clay is to remove all contaminates if you don't your just sealing in micro fine muck, also every time you polish a washed car you've not clay'd your putting tiny scratches into the paint, this shows up as the deadly dreaded swirls, and before you say "mine doesn't have swirls or marks" go out in the dark after washing it with a torch and shine down the paint and see 
