I don't see a reason to block cloaked users. You can't see what they are doing, they can't see what you are doing, there should be peace... You can send messages to them, so why not allow them to send too. IMHO it's very impolite to send a message to someone without giving him/her a chance to respond. I know what I am talking about, it happened to me too and it was not a good feeling.

OK, people of danoschek's kind might counter cloaked users could see what you are doing when they switch to guest mode, but hardly someone will really use this advantage May be guests shouldn't see users activities then and it would be really balanced.