Nor does getting stuff done require Ruby, or a Mac. It's just that sometimes it's nice to get things done in a nice way.
Plus the latest release of XCode, which MacRuby is preferred to use, supports Git natively.
I don't think this is so much about SVN vs Git, I think it's more about bringing MacRuby development to where the center of Ruby development attention is.
More people will click around new source commits and become inspired to make their own contributions.
Edit: my point was that sometimes keeping your focus on your problem, not your tools, yields the most gains.