Evidently you've never tried to write something like "Rear Light" one-handed on the iPad keyboard. Or, you don't have ogre fingers like I do.

More seriously, the pen is pretty fantastic for things like reviewing papers. Having used tablet PCs for years and now having used an iPad for a while, I can say it takes significantly longer to annotate PDFs via iAnnotate or goodreader and either my finger or the iPad keyboard than it did with Acrobat and the pen+tablet pc input panel.