I was curious as to the same question.
What I found out from various sources, RC and a Very reliable LFS, is the a UV is good, kills some bad stuff -- also kills some good stuff, but not enough to not use it.
I think you'll find more people use it than not. It will not hurt fish or corals. It might zap pods as things like that, but enough will get into your tank and be in your tank so not to worryl.
I;m not an expert, but this was my research. I use a 25W on my tnak, 24/7. Everything is healthy and happy.