I wouldn't mind this. We could even set it up so that a message displays like the one in /away. I mean if we're just going to tell that person to not bother us anyways, it would save us the time of telling people clearly why we do not want to talk with them. I do not think it would harm the community, because it is more harmful to have people bothered when they do not want to be bothered. Their responses may be blunted after continuous interruptions, and their reply may become increasingly impatient and rude.
I believe my /Away changes idea would solve this issue to some degree, but there's no harm in restricting the ability of someone to contact you in private. Also if you gave the contactee the ability to allow pm contact with users through user options or by a PM invitation (another good idea), it wouldn't really be restricting users from contacting you, it would change the process to them asking you first, and you deciding whether you want to chat with the user in private.