If that's the case, then why do we still have the thing at all? George Williams and Randalph Nesse came up with a brilliantly plausible explanation. Could it, they wondered, be paradoxically maintained by appendicitis?
The long, thin shape of the appendix makes it vulnerable when inflammation causes swelling that squeezes the artery to the appendix and cuts off its only blood supply. When filled with bacteria, an appendix without a blood supply cannot defend itself. Bacteria grow rapidly and eventually burst the appendix, spreading infection and toxins throughout the abdominal cavity. A bit of inflammation and swelling is less likely to disrupt the blood supply of a large appendix than that of a long, thin one. Natural selection gradually reduces the size of the useless appendix, but any appendix narrower than a certain diameter becomes more vulnerable to appendicitis. Thus, deaths from appendicitis may paradoxically select for a slightly larger appendix, maintaining this less-than-useless trait. Selection is also almost certainly very slowly making the appendix shorter, but in the meantime the appendix may be maintained by the shortsightedness of natural selection.
Thus according to this theory, the appendix hasn't completely gone away because past a certain point, any gradual effort in this direction results in more appendicitis. Back in the day, appendicitis was often deadly, and thus those people with a small, but not too small, appendix were at the greatest survival advantage.
The quote, by the way, is from Nesse and Williams' book, "Why We Get Sick", which is such a brilliant fusion of evolution and medicine that it should be prescribed reading for every undergraduate. We'll have more from it at a later date.