If it is the idler pulley, yes, it is a problem. The belt will "glide" over it, eventually "glazing" the belt and not be able to run the front end accesories (a/c, alternator, power steering, WP for zetec). If it is the tensioner, your front end accessories will not function properly due to lack of tension.....i was looking at my engine today and found that the next pulley up after the alternator is not spinning,the small one, the belt just rides over it.....does anyone else think this is a problem?