Paul Kedrosky champions it:
We want people to leave. We want cities and counties to constantly re-sort, to react adaptively, almost biologically, to changing economic conditions. The best way for that to happen is for there to be large population flux—inbound and outbound—and generally more people leaving than staying.
Mobility creates flux, and that drives energy and human collisions. The result is vibrancy, entrepreneurship and economic energy, or the lack thereof. Mobility is the economic social network that matters.
(Map of San Francisco county where blue lines represent inbound migration and red lines, outbound migration. You can adjust Forbes' interactive map here.)
The Dish 