I just added a short warning on my profile. The reason I did was that I see blogs that say they have downloaded pics on the web and if anyone wants them removed to let them know. Will it keep me safe? No. Will it stop someone who is kind and considerate (which I believe still exist "out there")? Yes.
Not much to add to the previous comments, except that much of those notices are crap, as I have seen warnings on pics that the poster definitely has no copyright ownership to. Some people think such matters, others try to add credibility with them, a few may think it looks cool to have such disclaimer and so on, so on.