I still believe this false distinction of "tablets must have a new OS" is false, only an artifact of current development. The way I see it, there is absolutely nothing that stops a "tablet pc" from being a legit PC except that apparently people LIKE being locked down into the OS. The rest is just a size factor. Supposedly Motorola had a prototype last year of a docking tablet, but all newcomers (including MS!) have to contend with an Apple-Android World.
My desktop happens to be larger and so the larger components have better fail-safe ratios, but in front of it is a wireless keyboard and mouse, so there is absolutely nothing stopping a tablet (or even a phone!!) from being a legit comp. It's all size vs stability of parts.
So Tablet OS'es are because apparently the mass populace LIKES being locked in, because most of their hard decisions are taken away from them in those use cases when they "just want to check email, the weather, and FarmVille".