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.

The operative word here being "most." You're right that for apps that meet those constraints, this is not especially compelling; the target here is for apps that are exceptions. We have produced cross-platform apps that require additional capabilities beyond what Phonegap includes, and have ended up writing a cross-platform UI, plus some native ObjC and Java glue for iOS and Android, respectively, to provide that capacity. The combination presented here would mean that at least on iOS, you could have a slightly more-pleasant glue-writing experience, particularly since anecdotally, ObjC is the hardest of the pieces of that kind of project for non-experts to get started with.