Since I find myself – for the first time in twenty-one years of reading The Straight Dope – having to question the validity of a given answer, I’m glad at least that the answer is not one that came directly from Cecil (no offense, Euty).
While it may be logical to assume that the practice of deleting a digit from the hand of an animated character came about as a time-saving measure during the animation process, such an assumption does not take into account the fact that newspaper strip cartoonists had been drawing characters with only three fingers and a thumb for years before the production of animated cartoons became commonplace.
For instance, as far back as 1913, George Herriman was drawing the characterss in his famous strip Krazy Kat with only four digits per hand (and no gloves). Many other early comic strip artists did the same. The true reason for ignoring the little finger may be lost in the mists of time, but some believe it had less to do with saving time than with aesthetics: the three-fingered hand just looked better on “cartooned” characters.