Xavier Onassis Bollocks. The point he was making was absolutely crystal clear and in no way diminished by his use of English unless your priorities are idiotically out of whack. ..., this is someone just talking in his perfectly understandable idiom.
Yes. It's perfectly understandable, but makes the person seem a bit thick. Views of people who are a bit thick tend to be taken less seriously.
Why is it any different on the internet, except that you can get away with sneering at people in a way you wouldn't to their face? It's rude and pointless.
If someone on here started making racist comments, or was boasting about crowd violence etc, you'd probably have got at them. In real life, you probably wouldn't.
Also "should of" is something that doesn't really come across in speech. It does grate when seen written down though. And it's not "just their idiom" at all. It's not slang or even lazy speaking. It's just wrong, and the result of people somehow not knowing that "should've" is a contraction of should have rather than being should of.