Objective-C is a fine language and is pretty easy to learn. Nevertheless there are a few advantages to RubyMotion for certain developers:

* No XCode. Getting comfortable with Xcode takes time and I frequently hear people complain about it crashing, being bloated, etc. With RubyMotion you can just use your regular text editor and a command line.

* Ruby. While Objective-C isn't bad, I actually enjoy using Ruby. I'm much more likely to refactor and abstract functionality.

* Third-party gems and code: While there are limitations, it is generally pretty easy to use gems and other ruby code in RubyMotion. From what I understand, you can also call out to third-party Objective-C libraries from within RubyMotion, so you don't lose out on that ability.

* REPL: Using a terminal and the simulator you can interact with your code while its running.