“But, O, it don’t matter!” is from Bleak House by Charles Dickens. It was spoken by Richard Carstone, a character who was studying law and medicine and, as McWhorter says, generally used "perfectly blackboard-style English."
“I wish you was here” was quite acceptable when John Adams wrote to Abigail from France in 1778. My English teachers taught me to say "were" because that's the current fashion, or perhaps I should say it was the current fashion half a century ago.
Go read McWhorter's article. I think you'll enjoy it.
But don't tell me grammar don't matter!
(I cross-posted this on my Joyful Noiseletter blog.)