the added value of a Windows 8 tablet over a Windows Phone device is that a heavily-encouraged subset of applications will run across those devices and a traditional desktop. Windows Phone will only run desktop Silverlight applications out-of-the-box (and even those require some cajoling.) Windows 8 Metro-style apps will run across desktops and tablets without any additional work.

and 90% of the world still runs Windows on their "traditional" computers, so this is an appealing proposition.