That's a lazy person's excuse which only serves to perpetuate bad behaviors. With less attempts at using proper spelling and grammar the chances of being misunderstood increases. Eventually reaching a point where the subject is lost in the confusion of being forced to transpose and parse the poorly written text to render it understandable.
I think you're reaching a bit. maybe to some, getting their message across is just more important than caring about a misspell or grammar. because as said, it's a damn internet forum where you talk to strangers. I admit, I understand how annoying it can be. but what's more annoying is the spelling and grammar police to be honest.
a lot of times, people will correct others just to make others look bad and in turn make them look better. just look at the post above me