My problem with the prescriptivists is not that they "believe that linguistic and grammatical rules are good", but that they sort existing linguistic rules into two categories: the "real" rules which everybody ought to follow and the "non-" rules that are "subpar". If this activity were not bad enough, they also "use" grammar (in some non-linguistic sense of the word), logic, and history to attempt to bolster their peculiar bagging of "good" (or perhaps I should say "only") rules. For example, not splitting infinitives or using which/that in certain kinds of relative clauses. So, it is not so much the rules that one uses, but the ones that one excludes that rile the descriptivists.
When somebody tells me I am "wrong" because of how (and not what) I said, I take them as fighting words, and begin the (verbal) beating it up machine.
Mary beat Johnny up.
Mary beat up Johnny.
"Subpar"! Snort, snicker, guffaw.
