I agree with you. I am seriously annoyed by the kind of mistakes you've listed, and it has nothing to do with the message being understandable or not. Instead, it means to me that the writer (often a native English speaker) has never bothered to master even the most basic rules of his own language, which in turn suggests a pretty poor education, at least as far as reading and writing skills go. Also, since he has never noticed his mistakes, he's either poorly literate, or displays a remarkable lack of curiosity and observation abilities.

On the other hand, I'm Italian. I came to understand, after a few years living in an English speaking country, that the emphasis Italians put on writing correctly (as well as on many other things - e.g. eating well, dressing decently, avoiding public drunkenness) has no correspondence in the English speaking world.

Did I annoy you enough? :)