I have units with them and without them. Prefer to have them. Kind of a belt and suspenders when it comes to water protection. I figure the less water against the seal the better off I am. Also on full sun camping it helps keep the heat off the top of the slide. Never had too much trouble with stuff gathering on them, but a portable ladder and broom or battery powered blower work wonders.
I will second the recommendation for the metal covers when the awning is rolled up. Makes them last a lot longer.
Aaron
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