I used to have a team of people who wrote commercial proposals - bids to get work. The standard of English was quite often abysmal, and I made a big deal of it.
The response tended to be 'everyone knows what it means. No one cares about this stuff except people like you...'
To which I'd reply, "The person who reads this might be someone like me. And I'm telling you, I wouldn't read past page one. You've put three weeks' work into this. Why risk it getting tossed aside because you've used 'there' when you mean 'their'?"
It always used to confuse me that people would press so hard to get training to improve their skills in all sorts of areas - presentations, Powerpoint, new technology - but if you suggested that they might like to go on a course to improve their written English, they'd get very indignant, or dismissive, or full-on offended.
The reacton was very 'bolshy adolescent' - which was a clue, I think.
When you criticise someone's use of English, you often trigger a repressed memory of being humiliated at school by some patronising teacher who made you feel stupid.
"Should OF? How many times do I have to tell you? This is primary school stuff!"
Not all teachers, obviously. But enough that a lot of adults simply want nothing to do with improving their written English, because the hell with Mr fucking Edwards.
I mean, in some ways, fair enough. I feel the same about swimming because of Lumpy Lampeter.