I always thought Microsoft were crowbarring their mobile UI in to the desktop system not because it was a good idea, but to gain familiarity. If all Windows users are familiar with the Tile UI, they might then be more comfortable getting a Windows phone or tablet - areas MS need all the help they can get.
On the other hand though touchscreen laptops and desktops are gradually becoming more common, so the fat-finger UI of the start screen tiles can work for that as well.
On the other hand though touchscreen laptops and desktops are gradually becoming more common, so the fat-finger UI of the start screen tiles can work for that as well.