Nice Q! I wrote a big huge response and it ended up as a blog post. Umm, so there's that. :)
http://thesoundmyheadmakes.blogspot.com/2012/07/who-can-wear-sound-designer-title.html
basic premise was that titles both imply skills and imply both what you can and can't do. I don't think the term "sound designer" carries much caché anymore, but it does serverserve a basic utility as a kind of shorthand to differentiate us from other audio guys like composers, location recordists, mixers, etc.