A recent article in the Boston Globe by Britt Peterson, "Why we love the language police," along with comments it has prompted on Facebook and other venues, shows that some people have become dangerously overstimulated by the publication of N.M. Gwynne's Gwynne's Grammar
Though I have not read it, citations from it in various articles indicate that Mr. Gwynne, a Briton, determinedly advocates for all the hoariest language superstitions and shibboleths (split infinitive, stranded prepositions, you name it) while insisting that observing them is essential to preserving the fabric of civilization. 
Mr. Gwynne's book is far from the first to deliver a load of codswallop about the English language, but it appears to have excited the peeveverein into commenting. Responding to their comments is wearying, so I propose this remedy:
What follows are some of the standard peevish arguments, each with a riposte. When you encounter one, instead of laboriously tapping out a reply, simply link to this post and cite the relevant number. (Use "Cf." That will impress.)
Number 1: It's a rule.
Thoreau wrote, "Any fool can make a rule, and every fool will mind it." And not all things that people call rules are in fact rules. It's useful to observe distinctions between rules and conventions. And further, many of these supposed rules have been manufactured out of whole cloth, like the journalistic over/more than superstition that even the Associated Press Stylebook has abandoned. 
Number 2: The usage is incorrect.
Usually this means that the usage is not generally accepted in standard written English. But standard written English is only one dialect of the language. You probably don't speak to your spouse, your child, or your pet exclusively in standard written English (unless you are an utter prat). Conversational and colloquial English are legitimate variants of the language. African-American vernacular English (commonly called "black English") is a dialect with a distinctive vocabulary and grammar; it is not erroneous English. 
Number 3: You're saying that anything goes.
This is the most familiar straw man argument. Nobody is saying that. What we are saying is that English has various registers and dialects, and that what is appropriate in one may not work in another. And while yes, people do make errors in English usage, many things that are called errors have been exploded as superstitions. 
What we are saying is that empirical evidence, the evidence accumulated by linguists and lexicographers of how people actually speak and write (including the work of reputable authors) is a better indicator of the actual rules and conventions of English, of good, respectable English, than a miscellaneous collection of schoolroom superstitions. 
Number 4: You're using standard written English, aren't you? Hypocrite. 
It's no surprise that editors, linguists, and lexicographers writing for publication use standard written English, the dialect most commonly appropriate for the intended audience and the publication. To say that African-American vernacular English is a legitimate dialect of the language, for example, does not mean that one is thereby compelled to use it.  
Number 5: It's not what I was taught
Some misguided or oversimplified advice you were given in the fifth grade may have been adequate for that time, but you're all grown up now and should be able to wield the language with adult sophistication. 
Number 6. You're degrading the language. 
There is no reason to think that English is on the decline. It's true that we can no longer read Chaucer without extensive glosses, and that is increasingly the case with Shakespeare, but that hardly means that contemporary English is degenerate. And no, young people's slang and text messaging are not harbingers of a time when we will communicate exclusively in grunts and whistles. 
Number 7. I don't like that word/phrase/usage/construction.
At last a point on which we can agree. It's your language, too. You don't like that word/phrase/usage/construction? Don't use it. 
What you don't get to do is pretend that your individual aesthetic preference is a law of the language binding on all users.