I have something similar as a shell script which can take a file as argument or paste its stdin, but it is for a personal pastebin (well it is public but I don't see the necessity of advertising it).
Lots of use cases. For example, I use Stackoverflow a lot and I need to paste code rather than faff around with pastebin. I can see myself using this to get a gist URL in one click.
That's a strange comment. OS X does indeed ship with Ruby. Maybe you are thinking about Rails? Or how developers often update it to a different version before using it? Mac OS X Mavericks includes Ruby 2.0.0p247.
In spite of the fact that Ruby is indeed standard on Mac OS X, a simple shell script would have been clearly lighter. However, for proper JSON escaping of the snippet, you need a full set of replacing rules, which kind of kills the purpose. So I give up.
I found this one which does something quite similar: http://sprunge.us/