Regardless of whether we *need* to generate our own unique ID, I'm leaning towards the thought that we're going to *want* to generate our own for usability reasons. In a perfect world, i think we'd have a sequence number so I could visit http://example.com/mailman/ archives/listname/204.html and know that 205.html would be the next message to that list, but any short unique id would do if sequence numbers are too much of a pain.
I agree there's a lot of usability benefits from short URLs, but perhaps this is the job of the archive server, and not the list server. Mharc (an archive server) is a great example here. Mharc's canonical message format is pretty human friendly.
Unfortunately, there's no trivial way for the list server to know that human friendly URL when the message is sent out. Fortunately, Mharc is also happy handles messages by message-id, which the list server does know about.
Had I been the implementer, I'd probably have made mharc do an HTTP 302 redirect from the longer URL to the shorter URL. But that's besides the point. The point is we have an existing, working, happy archival server, and it would be really nice if list servers (such as mailman) were compatible. And by compatible, I mean offering the capability of embedding an archival URL in the footers of messages.