There was a brief window when it was possible to do some really cool stuff with MacRuby, such as inspecting and even monkey patching core OS X classes. Probably one major reason why Apple killed the project in the first place. Being able to interact with the operating system on a low level through macirb was an absolutely mind blowing experience.
Sadly, those days are long gone, and I just don’t think Swift brings the same kind of flexibility or joy that MacRuby did.