The problem with prescriptive grammar is it doesn't always reflect the grammar as it's generally spoken. Although in very formal writing prescriptive grammar would be the better choice, I don't think it should always be the end. For example, if I said, "That's why we shouldn't go!" and, if I am generally and easily understood to be speaking a grammatical sentence, then that construction is acceptable.
I wouldn't have any problem with a speaker in a formal situation developing an argument, and pointedly ending it with the construction: "That is why we should not ________________." Now, had it been a formal argument in writing, I'd lean toward "The reason is _________________."
I don't mean to imply that simply because we can describe a grammatical construction, it is therefore acceptable in all situations. I do mean that when constructions are generally used and understood by most of the population, those constructions are acceptable in formal speech and even to some degree in formal writing. For example, consider the split infinitive. I don't think too many people worry these days about splitting it in formal writing as long as the split doesn't sound awkward and, in fact, facilitates the reading of the sentence.
