Not answering a question is Internet's way of walking away from a conversation. You don't usually say "excuse me, I'm late to a meeting", as interaction is asynchronous. In the current Internet culture, saying "I don't want to bother answering" sounds rude, and so the best solution signaling-wise is to just not answer.
In my experience (for myself and interacting with others), it's not uncommon at all for someone to
etc.
I declare this Open Thread open for discussion of Less Wrong topics that have not appeared in recent posts.