There's a lot to be said about the so-called "sharing economy." As Forbes' Tomio Geron explains in the current cover story, it isn't hard to make a connection between the current economy's scarcity of secure full-time work, and the appeal of services that can make anybody a couple of extra bucks by renting out things they already own (be it a bedroom or a power drill). But there's one effect of room-sharing sites such as Airbnb that is rarely discussed: These things can be insanely good for local neighborhood economies.