To be brutally honest, it's not an issue for me at the moment because there's nothing on my Windows Phone that I can't put on my iPad (that does have TV out via HDMI) or straight onto a memory stick and into my PS3/XBox just as easily if not moreso, and there's nothing unique on there that I'd want to see fullscreen either.
Presentations. Something like Keynote or Keynote Remote (and it's associated setup) would be *perfect* for me.
Video playing. Something similar to some of the WP7 + Xbox adverts I remember, where I could view video info while outputting video to/controlling the xbox. I do something similar with my iPhone + AppleTV combo, rather frequently. It's just convinient.
I'm actually considering getting a MBA13, soley for the convinience of presentations (which I have a lot of work in).