i used a clay bar on my jeep before winter and picked up a LOT of crap off my paint, its an 02 and probbaly the first time it was ever done. it felt like glass afterwards. not saying the paint was bad, it just was never clayed before, it was still shiny, but after i clayed it, wooooow did it shine
but after winter, i noticed some road tar and road crap on the bottom of the doors that wouldnt come off with washing it. so i used the clay bar starting on the top of the jeep, then working my way down. i didnt pick up anything from the molding up. but it took off all the crap that was stuck to the door from the molding down.
so i guess you dont really need to do it very often, but its nice to remove pesky specks that wont come off with just washing it