I always found tablet PCs to be either too big to be considered as mobile devices (PDAs are better for this) and not powerful enough to be real PCs (a real laptop is not that bigger).
But yesterday, I finally found what they're useful for: I had to write a letter with my own little hands, something I hadn't done for a looooong time. Ouch! It was physically difficult and I realized that I missed handwriting training.
So that's the real reason for tablet PCs: ensure that we, techno-geeks, have our daily training to still be able to use those prehistoric tools that are called