My at-work dictionary has a use note: "Traditionally few and fewer are used only before a plural noun few books; fewer people and less is used before a mass noun (less sugar)."
Not so traditionally. Let's see what God has to say about it. I haven't got this to hand to quote, and it's a bit long, but Fowler s.v. less notes that the word was formerly used very widely to mean 'smaller', 'minor', and so on (the greater light and the less, St James the Less), and the modern tendency is to use a more specific word: 'fewer', 'smaller', or whatever.
He does not mention the existence of a fetish "rule" that you have to use 'fewer'. Normally he vigorously attacks these fetish "rules". This suggests to me it's very recent, post-1930, or was very unimportant before then. All Fowler mildly says is that the use of 'fewer' rather than 'less' is the modern (c. 1930) tendency. Clearly, since then the tendency has foundered, since almost everyone says 'less' these days, and they have all through my life. I never heard of the fetish "rule" in school, though one or two of my teachers might have tried to force their pet fetishes on us. (One insisted that "human" wasn't a noun.)
There is a big difference between a real rule, learnt in infancy, such as plurality agreement or past tense formation, and a fictitious "rule" the existence of which surprises fluent adult speakers. Throwing out the fetishes doesn't change any of the real grammar of English.
