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%.