Firstly, and this forms the basis for my defence of all British/American language differences, the English language is itself the sum of the many languages that have contributed to it. And this is a good thing; the evolution of language is as beautiful as the language itself, and it is my belief that we should welcome these changes, not deride them. But aside from the wider argument, there is one very simple reason that Brits need to stop criticising the American use of -er, and it is this: the British do it themselves.
At various points in history, many of the -er spellings that are currently used by Brits (e.g. tender) would not have been considered proper by prescriptivists at the time. Actually, the word proper is a fine example of such a word; from Old French, it was originally spelled propre. Moreover, anyone who wanted to compose a written message to their zombie friend on the eleventh month of the year (doesn't everyone?) might have sent a lettre to the monstre in Novembre.
At times, Brits even use the -er ending on the word metre, especially when the word is used as a suffix. Consider, for example, the word thermometer.
If anything, what these discrepancies in British English show us is that American English—with the exception of words like massacre, mediocre, acre etc.—is the more consistent of the two when it comes to -re / -er endings.
The obvious point here is that, while you may accuse Americans of butchering the language, please remember that it was we, the British, who got the ball rolling a good number of years before American English was even a member of our esteemed language. My apologies, by the way, to the people of the late 13th century for misspelling membre.
This article was written by Laurence Brown. Laurence is a British expat living in Indianapolis, Indiana. He is a contributor for BBC America and has written for Anglotopia. He is Editor-in-chief of Lost in the Pond and loves nothing more than to share these articles with anglophiles, expats, and other interested parties on social media. Follow Lost in the Pond on Facebook, Twitter and Google+.