I do know there is a young lad in Sunderland who is very happy he put a £30 Hoya HMC UV filter on his £500 Canon EF 70-200mm f4L lens.
He was changing lenses yesterday morning when he dropped it. The lens cap was off and while the hood was on it was on backwards in its stored position. The front of the lens took the full impact, the filter cracked but once we had removed it the front element was in perfect condition.
Would the front element have been marked if the filter wasn't on? Don't know to be honest but I do know it wasn't with a filter on.
The reason he had the filter on wasn't actually to protect the front element or filter the light in any way it was because Canon says the lens is only weather resistant to their spec if a filter is in place. He had simply done what Canon said he should - and a very relieved chap he it too.