That's purely the personal preference of one curmudgeon from 1770. "The OED shows that less has been used of countables since the time of King Alfred the Great -- he used it that way in one of his own translations from Latin -- more than a thousand years ago (in about 888). So essentially less has been used of countables in English for just about as long as there has been a written English language."

http://itre.cis.upenn.edu/~myl/languagelog/archives/003775.h...