[flyingbeaver]

"If I have to learn all the Objective-C / Cocoa APIs anyway" and "who want to avoid learning a new $language"...

Still not the issue, sure some people want to avoid to learn more, but that's not people we are interested in, those people wouldn't learn anything anyways... As you said, if you want to be very good yes you have to learn all the tools. So for good developers it's not about learning this or that, it's about learning both, and choosing the one you think is the best to write a full application.

", idiomatic JS does not map cleanly to idiomatic ObjC, you end up with code that is idiomatic in neither."

Sure, you end up with idiomatic RubyMotion. Is that an issue ?