No good comes from (or is intended, as far as I am concerned) using a person's real name online when they are using a non-name ID. People do this for security purposes. The idea that one would use someone's name (because they have dealt directly with the person through the mails, thus REQUIRING a real name) to address them online is NOT polite, nor is it wanted, in the least, by most people.
If someone wants to be called 'Bob' at work, but 'Dave' at home, who's business is it to mix up the names at the two places? It strikes me as simply a way to 'out' a person when they don't want to be 'outed'. I've been lucky - no one uses my real name (mostly because **I** don't) online, which I prefer for security purposes. And isn't that nobody's business but my own?? Using an alias online is standard - everybody does it - and it is rarely done to be underhanded. To be 'outed' by someone else, though, ONLY appears to be underhanded.