[wprater]

A few reasons:

There is no build system in PhoneGap, just instructions on how to get it build with Xcode. Using the RubyMotion build tools will allow you to work around this, all you'd really need is the Rakefile in this case, no extra Ruby files.

It's agreed, that most may not need to write (or alter) Plugins for PhoneGap, but if you did, and if you'd prefer to write in Ruby, this article may be helpful!

Or maybe you just want to tinker around.

[endemic]

Hmm, I thought that v.2 shipped with some build scripts, so theoretically you wouldn't have to open Xcode, just do everything via command line.