a solution looking for a problem?
I think that's harsh. There are lots of very neat uses, the reason we haven't had them is it's really difficult to build not that they can't think what to use it in. In fact they've had years to plan uses while the tech is proved (a mate was working in this field out of uni, 15+ years ago).
Tablets are way better than phones for a lot of things, but they can't replace phones and you don't really want to carry a tablet 24/7, let alone a tablet AND a phone. The trend in ever-bigger phones/phablets shows there is a desire for a convenient, large-screen device.
A phone that can actually fold out into a tablet, without weighing a ton, would be great. I can't really see how they solve the battery or weight problem though even if the screen is market-ready.