My husband regularly commits all the grammatical sins listed in this thread, and more. His written and spoken English is dreadful - and he is the first to admit it. I'm the opposite. His accent is, well, Danny Dyer, to be honest, whereas mine is more RP.
I never felt the need to correct him but now we have a child. I don't specifically correct him by saying he is wrong but when he makes errors in his spoken English I repeat the phrase correctly (them/those, you were/you was). The thing is, this is what he wants. He feels that new people respond to us both very differently, he often feels people look down on him but treat me differently, he is very conscious that the way he presents himself does not convey his natural intelligence. I hate to say it but experience tells me he's right. He feels this puts him at a disadvantage and he would hate to see our daughter in the same position, so I gently model correct English usage to make both of us happy.
Furthermore, our household is trilingual and we live in a part of the world where daily life is full of encounters with people of a number of different nationalities and we have both noticed that using incorrect language makes it so much harder for non-native speakers of that language to understand what you are trying to say - be it English or any language. Correct expression makes all of our lives easier here.
And as for "I mean, at the end of the day, who gives a fuck, really?! confused", well I had to give a fuck. A few years ago now, in London, we took on a member of staff with a very, very broad northern accent - not a problem for me. However his speech was very, very heavily populated with his local dialect and included many grammatical errors.
Unfortunately our job involved dealing with many foreigners, on the telephone, in person and via email. Even those who spoke very good English really struggled to understand him. "Arrrrr fancy a brew?" was perfectly polite but meant absolutely nothing to non-native speakers, as one rather quaint example.
While in person these misunderstandings were easily corrected, this wasn't the case by telephone or email, and it got to the point where he couldn't be left on shift alone. My budget and staffing rosters became impossible to manage because I had to keep finding extra staff to translate this Englishman's English to non-native speakers...
We all suffered - he knew exactly what he was doing but in this politically correct world he gleefully asserted his pride in his local dialect and claimed discrimination if asked, for the sake of politeness, to moderate his speech in order to do his job properly. It was just bloody rude.
Sorry, that was really long!