I’ve written before about how spelling mistakes are not grammar mistakes, and gave an example of how you could test this. In the case of your/you’re confusion, no one ever tries to extract an auxiliary out of your. So even people who write “You’re parents are home” would never attempt to make a question like “Are you parents are home?”. People fundamentally know the difference between the possessive and the contraction, and it’s really just a simple spelling mistake.
Anyway, I think it’s time to revisit this issue, after reading a quiz on the Telegraph: How much of a grammar pedant are you?. (edit: this link now seems to be dead. Sorry!) I’m not a grammar pedant, but I am a picky linguist, and I don’t like this quiz. It’s supposed to be about grammar, but not all the questions are actually on that topic. They are mostly about spelling, punctuation, or writing style. First, I’ll give a quick overview of grammar vs. writing, and then I’ll tackle the quiz specifically.