Once upon a time, I worked on a college newspaper as copyeditor. This position entailed writing an editorial for each issue to help fill the page. I went to work, happily tearing down all the vices of consistently bad spelling, misused punctuation, and the dreaded they're/their/there problem.
The paper got a letter, from an education major who shall remain nameless (mainly because I can't remember, and am too lazy to look it up), riddled with spelling and grammar errors. It accused me of elitism, scoffed at all the bile I was wasting railing about something so silly as how stupid people sounded when they didn't even try to string together a sentence, and then pointed out all of the errors she'd found in the paper over the last several issues.
The letter taught me something, but not what the letter writer intended. It confirmed my suspicion that those most defensive that spelling and grammar don't matter are those who don't care they're bad at it. It told me that presentation does matter - after all, the errors she'd noticed were supposed to reflect poorly on me. And I learned that I was doing a good job; all of the errors the letter writer found were in sections I didn't proofread.
I'm more aware these days of the immense privilege I had in attending a school where some grammar basics were taught, where the teachers not only marked our papers in red ink but explained what was wrong with what we'd written. I understand that bad grammar is not a sign of lacking intelligence, or laziness.
But I understood that then, too. My point, which I still maintain, was that presentation matters. What you say, how you say it, and how much effort you put into polishing it all matter, sometimes more than what you're saying (or writing). This is true whether you're a writer or not.
As a writer, if you submit your draft for publication or for representation by an agent, and it's riddled with spelling errors, you're going to have a hard time selling that manuscript. From what I've heard about the publishing world, new manuscripts aren't competing with a majority of poorly-written dreck. Most submissions are cohesive, well-written, and polished. Of two similarly well-written stories, one that obviously needs more work on the basic level (as in, spelling and grammar) is less likely to be picked up. It's the same reason why they don't build houses on top of mounds of sand: the foundation has to be strong.
Grammar and spelling matter outside the publication world, as well. If you misspell a job title or description on your résumé, you're going to have a hard time getting a good look from an employer who recognizes the need for a professional presentation. (In my limited experience of HR offices, that's most of them.) Send an email riddled with sentence fragments and a bad grasp of apostrophes, and a business contact may go with someone who can put forth the effort to string sentences together properly, instead. You won't see businessmen coming to blows over the minutiae of grammar, but it does affect one's business life and how people perceive you.
That is not to say you're doomed, if you haven't grasped it by now. The beauty of the written language, and what I especially like about it, is that it can be edited. You can't take back having said something stupid, but you can stop and take a few moments to look your emails over for common spelling errors. You can have someone who knows these things proofread your résumé or cover letter. You can spend a few minutes every day familiarizing yourself with the basics, and vow to stomp out one faux pas at a time.
To me, it's always been about the effort, not the errors themselves. I'm not going to snark at every email I receive and make fun of every spelling error. I'm not going to forward a poorly-written email to my co-workers or fellow grammar police to make fun of the person who wrote it. I'm not going to correct a comment's spelling and grammar if I find it lacking.
If, on the other hand, I see a consistent pattern of someone making the same mistake again and again, and I know the person well enough, I'll try to correct the writer in a gentle and nonjudgmental way. If that person has made it clear this approach is unwelcome, I'll leave it be. I'll also correct my own errors, often loudly and blushing furiously as I do.
English is not an easy language. I get it. All I'm looking for in any written correspondence is that I'm worth the effort to attempt to communicate with effectively and meaningfully. If you don't care about grammar, that's your prerogative, but at least try to think about the message it sends.
The title of the post, by the way, comes from a printed tank top my ex-husband got me for Christmas one year. I also have a "Grammar Police" mug. Same source.