To me, it's absolutely rude not to respond to the messages, even if the person who sends you the messages is not a priority. I don't treat all people the same, but I have never ignored a single message or email or a phone call. Apparently, it's good to be rude in this world, and it seems to work best when you are One person told me once he doesn't respond to all messages because it makes him appear "important" and "busy"!! I suspect many people think the same.