I began using "theatre" over "theater" a few years ago, more as a means of demarcation than pretention. The English language is full of homonyms, and "theater" can mean the art form, the place where such an art form is performed, and a place where one can see Tom Hanks's face projected onto a giant screen.
So I switched back to the original "re" spelling when referring to the art form only. I keep to the now accepted Americanized misspellings for the venues.
And that's the truth of it. "Theatre" became "theater" through the phonetic spellings of the less educated. That's what happened to the "u" in color. People couldn't hear it, so they didn't write it. Eventually, dictionary makers gave up and said, "Fine! Both spellings are acceptable." Eventually the misspelling overtook the original correct spelling: a testament to the American philosophy of "Don't bother me with the details."
I worry that this will happen with more words as l33tspeak and an over-reliance on spellcheck further diminish the reverence that we have for our language.
Your pet peeve is that some people wish to adopt what is the original (correct) spelling of the word "theatre."
My language pet peeves include a growing disregard for the difference between "your" and "you're" and between "its" and "it's" and this new thing of writing "should of" and "could of" as a transcription of "should've" and "could've" (which, when written, really should go back to "should have" and "could have.")
It's laziness, and, sadly, the guardians of our language are more and more inclined to simply throw up their hands in defeat. (Not "there" hands or "they're" hands, mind you, but then fewer and fewer of us actually seem to care about that, either.)
I shudder to think that someday someone will be considered pretentious because they insist upon including all of the vowels in "srsly."