With clean cover glas, you'll loose 8% of your light output and 20% if it's dirty. At least that's what I've read somewhere in a Dutch forum. Offcourse if you have lot's of light and you clean it well/periodically I don't see any problems.

I don't know what kind of light you use, if it's T8/T5 there is those IP67 lamp holder. It's suppose to be water resistant even under (!) the water for a few moment. But ofcourse the best thing is to install your light so it won't fall off. If you don't like the cover you can either do this or teach your cat a lesson or two .