I am with a couple of them! But I totally get why someone wouldn't want that. Sometimes you need to keep that division between personal and professional.
My personal life is fairly tame, lol, and somewhat boring. So I am not worried about them seeing anything untoward. And if they did, I should hope that they understand that I am a real person, not a childcare robot!! Sometimes I go out with the girls, have a few drinks and a good time. There will not be any naked pics of me, or anything else I wouldn't want them to see. Just as I will not judge what I may see on their profile, I should hope the same.
And on the other hand, being friends with them has given me a little insight into their lives too. I don't think being facebook friends means we are "friends"-friends. It has proven a good way for me to communicate with some people, as well.