With respect to the original post, I resent the suggestion that I don't value the English language based on, say, responses to Mikey's thread. The primary issue is that I don't see how it's appropriate to correct someone about their perceived misuse of English a) over a phone call when conducting official business and b) when it's dialectal English rather than actually wrong. It just doesn't seem to be the right situation to do so. The comparisons to arithmetic errors, meanwhile, are just misplaced. Mathematics is either right, or wrong -- and anyway the mistakes affect money, something far more tangible than an abstract "double negative" issue (not even an issue in many languages anyway). Language evolves, and what is "right" now was "wrong" even as recently as 50 years ago, and vice versa, so correction or hypercorrection comes across as dogma rather than helpful, friendly advice.
As far as I'm concerned now, the best way to deal with English "errors" is to look after your own use of the language, while in talking with other people, the only time to query their use of language is if you genuinely don't understand what they are saying or meaning. In Mikey's phone call, that didn't happen -- it was obvious what she meant -- so there was no ambiguity in meaning and therefore no need to try and correct anything.
In point of fact, most of the points of grammar people seek to correct aren't actually rules anyway -- just un-necessary dogma from an unhealthy 18th-century obsession with trying to make English the new Latin.