Personally, I hide it because that's what society is telling me, especially if children are around, and I have no real reason to go against that. I mean, who wants to see what I do in the bathroom? But should the government want to, I will gladly let them as it will nicely illustrate what I think of them.
There are many things I want to hide more than my body functions. It is a social taboo, not something that has to do with personal safety and security, which is what privacy advocates usually point to. Arguably, it is the opposite problem: something you have to hide, but for personal freedom, you shouldn't have to.