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I don't get it: what advantage do they have over Apple here?
Because Apple had said multiple times that they fundamentally don't believe in a one device world. Tablets are tablets, desktops are desktops. Mixing the two bastardizes both. Microsoft doesn't agree, and Win8 is a hybrid that supports both interactions. Ubuntu is showing it will do the same. Personally, I think Apple are seriously short-sighted in their approach. The other possibility I they are being very long sigh…
Yes, why do one thing well when you can do two things adequately?
I think this fundamental difference separates Apple from nearly everyone else in the markets in which they compete.
Apple's vision is similar to Sun's: one ID that allows you to transport your data (incl. apps for appropriate platform) to all your Apple devices... Have you ever used a Sun-Ray[1]? It's like that, except for apps/data, not sessions.
Personally, I don't think Win8 is anything like Ubuntu for Android. Microsoft has the same problem as Apple: they don't want you to stop buying desktops, as both companies make a lot of money on continued sales of traditional computers. Whereas Apple created a whole new device platform in the iPad and iPhone, Microsoft is trying to merge the two markets back together... while at the same time relying on the computing profits to keep their overall margins up. Meanwhile Google and Canonical would happily cannibalize any revenue from desktops for the much greater potential in mobile.