I think a usernote is a little way to brighten someone's day. When you give someone a usernote, it makes them smile. If someone gets an offensive usernote, then can delete it, or they can tell a mod. I think the user can moderate them on their own.
That's just my stance on them. I like usernotes.
Usernotes other than the user's own cannot be moderated by the user. Only staff can delete usernotes, so they all have to be reported in some way if they are a problem.