In my observation I sort of admire what Apple is trying to do in re: making iOS and MacOS work in harmony with a similar look-and-feel, but they certainly aren't there yet--I consider my own and Wife's Macintoshes to be mostly just toys, though it IS nice that all of our contacts and email and messaging are available on all of our iPhones, iPads, and Macs.
As someone who just bought a bunch of UI/UX design books and HIGs with the intent to have citations handy while spare-time designing a HIG for the hypothetical 2025 expression of what Windows 9x, Mac OS 9, GNOME 2.x, KDE, etc. seemed to be converging toward (like how all consumer automobiles have basically the same layout for critical controls), I already know enough to know some of the problems with all this. (I mention those OSes because OSX broke its own HIG for glitz's sake, Windows 7 had The Ribbon, GNOME 3 is a wannabe tablet UI, etc.)
You can't converge mobile UIs and desktop UIs for the same reason you can't converge 2-foot (desktop/laptop) UIs and 10-foot (TV) UIs... because they're designed around different hardware constraints.
For example, the thin little auto-hiding scrollbars you see on mobile devices are objectively worse... except when every pixel on a tiny little, narrow, portrait oriented screen is precious and that trumps other concerns.
Big fat widgets with a ton of whitespace padding and/or big text are objectively worse as functional objects... unless you need to account for comparatively fat fingers on touch screens or humans who want to read the thing from 10 feet away without squinting.
Convergence is the UI equivalent of arguing that all coffee table books and children's books and magazines and so on should be held to the same material and dimensional constraints as pocket-book novels (i.e. cheap paper, black and white printing, 4.25" x 6.87" dimensions) for "consistency with" the portable option based on some fallacious "Mobile devices now outsell PCs massively. Let's try to be more like mobile devices and maybe we can get people to buy more PCs." logic.
It makes about as much sense as saying "Young people are drawn away from car ownership toward living in cities with good public transit and young people play video games. Let's try to attract them back by partnering with Microsoft or Sony to replace steering wheels, foot pedals, and gear-shift levers with XBox or Playstation gamepads."
(Granted, there IS also a strong element of wanting to get away with not having to make and maintain separate desktop and mobile UIs for applications.)
