Good to save critters from drowning in your rain barrels, if only to keep the water from getting really stinky. Skinky?
I have screening over all my rain barrels now, because the geckos were getting in there and drowning and stinking up the water. Keeps them from breeding mosquitoes, too.
It's sort of our fault, since they wouldn't get into trouble in rain barrels if we weren't collecting rain water . . .