I'm quite opposed to this. The benefit is negligible, and the cost is substantial.
The benefit, as I see it, is merely to eliminate some typing on user subpages, and to enable automatic backlinking on those same pages. This is a very tiny part of our overall mission, and so the benefit here is quite small.
The cost is that we are adding syntax in one part of wikipedia that new users will discover and come to expect throughout all of wikipedia. Increasing the befuddlement of newcomers is always a bad thing, although it can sometimes be overcome if there is a very very strong and compelling reason.
--Jimbo