The Quaker plain speech began as a way of protesting social rank. The early English Friends did a lot of jail time for their radical views, and their somewhat combative social identity was predicated on not deferring to people of higher rank with the pronoun "You," or condescending to those of lower rank with the more familiar "Thee." We were all meant to be "friends," after all.
It's still around today. More broadly, at issue is the Quaker testimony of "simplicity" in life and speech whereby (as Jesus admonished in the gospel) our yea should be yea, our nay, nay, and we shouldn't have to swear by God's name in order to prove our honesty. Titles conferring rank are discouraged, if sometimes necessary.
When I was 10, my family applied for passports to visit the English side of the family. It's hard to believe now, but the process involved some sort of government official in an office building in downtown Philadelphia, a Bible, and a discussion between my parents and said official about "swearing" versus "affirmation." In the end, we affirmed, rather than swore, something that made the government okay with letting us out of the country. This is, apparently, protected in the Constitution, no doubt having to do with the influence of early Quakers and other non-conformist religious groups. I can't remember if we touched the Bible or not. If we did, I think it was a concession to The Man on my parents' part. (The first time I heard anyone use the word "idolatry"--in a shocked and disapproving tone--was when I came home from tennis camp and told my mother that we kids had to recite the Pledge of Allegiance every morning.)
My English grandmother, who lived near us in Philadelphia, used the plain speech all the time. It was so much a part of her that I can't remember how she handled non-Quakers--increasingly, most of the people in our lives. She, and other plain speakers I've encountered, used a non-grammatical form of "thee and thou" that was once common in the North of England, where, incidentally, she grew up. So the fancy-schmancy "dost thou want tea?" never entered my ears. Rather, "Does thee want tea?' Nice and homely.
When I was about ten and my brother twelve, our mother embarked on a doomed Plain Speech Crusade in our household. We were too old; we were unwilling to be dragged backward in time. "We Quakers are a queer lot," my grandfather complained in a 1920 letter to his father, and my brother and I didn't want to feel queer-er than we already did. I considered myself locked in a time warp in a house dominated by the somber tick and chime of various clocks made from dark wood that were tended and wound up like family members.
"Baby, it's you," the Beatles sang on my rickety record player in those days. "Nothing you can do but you can learn to be you in time." "You say you want a revolution..." And when Bob Dylan told me, "Whatever colors you have in your mind/I'll show them to you and you'll see them shine," it just wouldn't have been the same, rendered in plain speech.