Why? Nothing forces you to write to people you don't like... It can sometimes be convenient to have "not seen" a particular person's request.
I only worry about total lack of communication over such things. A complete blank wall in a game/system where it's incredibly hard to see if someone is doing anything at all can be incredibly frustrating. I understand that sometimes you need to "not see" a letter to buy time for something else, but if people make a habit of it as a way to just not do what people want then it can be quite frustrating. I've got one region lord that I'm starting to jack up their duchy tax because they're "not seeing" any letters anyone sends them, though they keep wandering around. If they just wrote back I would be more than happy to not tax the pants off them.
I do wonder how high they'll let me get their duchy tax before they deign to respond, though. Maybe I should take bets.