The point isn't that there's no point in grammar and we should all use it incorrectly, it's that we shouldn't penalise and judge people who make common grammar or spelling mistakes, especially in informal settings. They are most likely not trying to communicate on the level of an academic paper and they don't need to. Either education or natural ability is so linked with peoples class or disabilities, does it make you feel good to put down people when you don't know a thing about them beyond that they mix up there/their/they're? Besides, common 'mistakes' in langauge is often just the language itself evolving. If multiple people keep making a mistake again and again that shows there's something about the grammar or spelling thats confusing or unprefferable to people brains, and what you now think of as a mistake could be the standard for the future. So much of our language today is based on mistakes people made in the past, nobody sat down and wrote a big rule book on how to speak and invented langauge, it evolves from its speakers.