JazzO sez: using "less" there is absolutely wrong
What it boils down to is whether the native speaker of the language sees any meaningful difference between the situation in which what is being talked about is measurable and that in which it is countable. If the speaker sees no such difference then it makes no sense to use a different word in one instance than in the other. Less is easier to say than fewer and therefore is preferable to the native speaker.
There are many rules that used to be used, e.g., separate verb endings for each person and number, use of a different pronouns for plural and singular in the second and even different forms of the pronouns for nominative (now usually called subjective) and dative/accusative (now usually called objective) in the second person plural (it's not who do you trust or whom do you trust, it's whom do ye* trust). All these rules were forgotten because they were seen as being unnecessary. I regret the loss of meaningful distinctions such as the difference between virulent and virile or enormity and enormousness but I refuse to mourn the loss of a distinction between less and fewer.
Show me a case where there could be a misunderstanding based on the violation of this rule and I may recant.
*Ænigma doesn't even recognize ye.
