Though you are right, Mac viruses are few and far between...I just ran into SevenDust the other day...and a buddy of mine just had *all* of his extensions deleted via its nastiness as he didn't have Norton to back him up. Whether you're on a Mac or a PC, antivirus software is a must.

As for the what the virus definitions add, just read that "what's new" text file included with each update. A lot of times, it's updating against the various, low-level macro viruses that can still get Mac users who are using Microsoft products(MS Office, etc)...