Handwriting is still a common form of communication for a lot of people.
While people like ourselves are used to a keyboard and can type ridiculously quickly, many computer users out there still have problems.
A Touch-Screen would also alleviate the need for a Pointing device, pushing an onscreen button with your finger is lots more intuitive than scratching a little white pad below the keyboard to make a tiny white pointer move over the button, then pressing a button below the pad.
When I go wardriving, for instance, or need to dial a number to my bluetooth phone from Address Book, whilst Driving, am often Frustrated by having to move the Mouse Cursor. On occasion I've found myself tapping the image on the screen, before scratching on the TrackPad.
FInally, when compared with (IMO) frivalous tasks like voice recognition, the Processing required for reliable handwriting recognition would ba a drop-in-the-ocean.