With Content Sizes allowing us to use 1 Storyboard for iPad/iPhone and other once iPad-only views (UISplitViewController) now on the iPhone, I can see a time when we have 1 codebase for Mac, iPad, and iPhone.
I wouldn't be surprised if Apple added a "Large" content size for use on stuff like the iPad Pro (?) and the Mac. I think there is lots of value to this as there are far more iOS developers than OS X developers and hopefully Mac development will see an uptick.