Well - Mine have been off for 3 1/2 years. So far all is fine. But I have found more crud under the back bumper area. I just try to wash it more often is all. I don't think it is a big issue.

Now just before I took mine off, I found that snow would build up between the flaps and tire more than they do without the flaps. I would have to kick the snow out before I drove off.

With them on, it does help a little to keep crud off the sides. (oops that was just said)