"If MacRuby was a public part of OS X, developers writing programs with MacRuby wouldn't have to ship the MacRuby runtime. On the flipside, developers would also be tied to the MacRuby version shipping with Mac OS X ..."
Why couldn't an application bundle its own ruby runtime and skip using the built-in version?
I think the point is to reduce the app download size by using the built-in MacRuby already installed. Chances are, when you ship your application, it's several times larger because you have to bundle the Ruby runtime.