It's probably a sad reflection of society that thinks anything on the web is free for them to use, however they want. A copyright notice doesn't prevent theft, but it does make the theft require a bit more effort, so may deter some casual thieves and it also reminds people that it is in fact copyrighted, as there is also a misconception among many, that it is only copyrighted if it says so. That's certainly the compromise I have made between having my work visible and making it slightly harder to steal. It doesn't stop a determined thief, but then very little would.