I think traditional guestbooks are unnecessary - obviously you know who is there from the RSVPs and you know their addresses because you send them an invitation. That said, I think a non-traditional guestbook is wonderful, especially for a small wedding. You can have something that allows people to write you a brief note or do something more personal.
I bought a cardboard spiral book from oriental trading company and sort of "scrapbooked" it with pictures of my fiance and I as well as ample space for guests to sign and write a note.