I care about proper writing for the same reason Van Halen made that concert rider that demanded a bowl of M&Ms with the brown ones all picked out. And sure, to a certain degree, it doesn't matter, just like the color of candies in a bowl doesn't matter. If a student turns in a paper without Oxford commas or with sentences that end in prepositions, yawn, big deal; that's an honest difference of opinion about correctness, not the end of the world. Even if a student turns in a paper that talks about the Hindu's and the Rasta's and creates run-on sentences by using commas where semicolons should be, I'm pretty willing to let that slide as a reflection of a student's writing to a perceived (if not actual) level of correctness.
But when students turn in things that misspell key words repeatedly, include nonsensical spell-check substitutions, and/or have whole sentences that are just garbage, it's like seeing the brown M&Ms: I know they were careless, and it's a safe bet the rest of their paper is going to be equally careless
ladysisyphus explains why spelling and grammar are critical. With Van Halen and Ebonics!
Although I wish to add: The Oxford Comma is always and eternally invariably correct, and knowing failure to use it is abomination punishable by death, or at least a failing mark. The delimiter of a comma-delimited list is a comma. In the fake and obviously-manufactured constructed situations where the comma-delimited comma-delimited-list can be unclear, it is also obviously clear that rewriting the sentence would be better AND that the situation was constructed specifically to create a situtation where comma-delimiting a comma-delimited list might be confusing.
Students who do that get a pummelling.
For the record.