While you could argue that this is the result of desensitization, I personally don't care enough to complain about its use, or even see anything necessarily objectionable about it. Is it possibly an insensitive way to express strong feelings that one could find other ways of letting out? I guess. However, I find that life is a lot easier when you don't take harsh words to heart. Not saying that everyone is that stone-hearted or even should be that way, but a little stoicism goes a long way. If someone insults you, prove them wrong with your actions, at the very least by punching back. Where I live, if someone calls you a ret*d, chances are there's already a fight, so might as well defend yourself. And whether or not actually mentally challenged people are offended by its use, well, it's not my place to get offended on the behalf of others.