My main inspiration of thought along these lines is the urge to want to post things like photos online for friends/family, but I shy away from using something like Facebook to do it. The only problem with 'rolling my own' is the I don't want to necessarily make all photos world-viewable, and I don't want to force my friends/family to create an account just to view my photos.

If all I had to do was grab their public key, I could then do everything behind the scenes (even creating groups so that some people can see some photos and others can't).