I was with you until: "So if something is painful to write in Objective-C, just write it in Ruby and easily add it to your project. Super neat stuff."
Nope, sorry. If you're going to write a native app, then write a native app. I abhor ob-c, but I'd MUCH rather not have a patchwork of languages to maintain on an iOS specific codebase.
Writing in RubyMotion is writing a native app. It uses the same object model, the same runtime, and the same libraries as Obj-C.
The only problem with this is that since it's a static library you need to make sure it's bug free before using it in your project. I would imagine it would be difficult to debug in your project...
There are petty crimes and there are serious crimes. Objective-C in Ruby is definitely the latter.