You definitely do not NEED one. That said, it may come in handy from time to time. I think it depends whether or not you're particularly attached to having hard copies of handouts, notes, etc.
Last semester, the text to one of my classes was in pre-publication stage, so the prof made it available to us in pdf form. Having the tablet, I was able to mark up and highlight the pdf, all with out printing out the text, saving me almost $100, which was cool. Also, I find it convenient to be able to annotate Lexis/WL print-outs right on the computer. Finally, sometimes it is useful to be able to quickly draw a graph in your notes, although I guess you could just as soon bring along an extra notebook for those rare instances.
One other thing -- Microsoft OneNote on a tablet is an awesome way to organize your notes. I totally recommend it, even if you don't have a tablet PC, as you can cut and paste things anywhere, and save notes in pages and tabs, just like an actual notebook.
HTH. Good luck this coming year.