The biggest problem that MS have is that although all their products share a common-ish theme (The tiles) the codebases are not compatible so they lose a lot of developer support. Their strategy is reportedly to converge on a single code base for all devices. If they can pull this off then it will be worth the while of developers to write more apps for them.
Of the four main consumer product lines, XBox has its niche, full blown X86 Windows will be around for lifetimes yet and the phones are pretty sweet (and starting to take off now, at least outside the US) RT, however is really a bit pointless, especially considering, as others here have pointed out, the affordability of decent Atom-based tablets.
The tiles work well on a phone screen, I have one and I wouldn't go back to an iPhone now. The first part of the convergence will be to merge RT and Phone OSs, with features applied dependent upon screen size. The 'RT' brand will then quietly fade away...