I was just reading Hemmingway today. His writing was good. It was not fancy. His sentences were short. I like the short sentences. But then his sentences could also be long and go on for a time without a comma until one came, and then there would be another, and then it would end. It was a strong way to write. It was a strong way to say something. In everyone's writing there is a voice. IMHO it's worth trying to use proper grammar, but writing is expression, and sometimes being too conscious of grammar quiets that voice, which is a bad thing. I appreciate Hemmingway because although he usually used proper grammar, it didn't get in his way. He just put periods and commas where they ought to be, and that was that. I dislike when an effort to use proper grammar gets me bogged down in an unweildy sentence. One thing I appreciate about online discussion is that it gives me an opportunity to exercise my writing. But I don't really care if people use proper grammar or not. Usually their ideas come through, and I can always ask for clarification.
I think we write because we want to communicate (and connect with others). We don't want our readers to be distracted by obvious spelling/grammar errors. We hope our readers will focus on the message, but sometimes they become fixated on an error and lose the message. Maybe they offer suggestions for corrections because they want our message to be more perfect. It's complicated. I would want to know if I made an error in an online posting because I feel errors would weaken my credibility as a writer. Would you want to know?
I think its different on a composed blog post and an informal comments section. If you want to be taken seriously as a writer, then sure, have your act together. On a comments section, I find that the rules are (and should be) a lot less formal--conversational, perhaps. The only people who seem to correct others in a comments section are trolls, and people who are devoid of quality arguments. Now if you're talking about errors of judgement or factual errors, then I'm with you 100%.