I think if you're self diagnosing with a view to getting treatment only a professional can sanction - then i don't really see the harm. But usually to access that treatment you have to be professionally diagnosed. That's how it works here. I think it's great to be as clued up as possible especially if you go and see someone who has malpractice down to a fine art...but with that comes frustration, especially if you know you can't access what you think you will help without someone listening seriously to your concerns. If you're using self diagnosis as a means to try and justify certain behaviours or to deny responsibility for your actions then i have a problem. It slowly wrecks that persons life even more (usually without them seeing it) and what does it do for the stigma and difficulties faced by people who actually have that disorder and are trying as hard as they can to change?

A lot of variables but regardless of who gives you a label, if you don't intend to do anything constructive as a result of it - i just don't see the point.