I guess that has advantages protecting against the elements, but then I wonder if you lose a lot of the charm of open air streets? Al fresco dining (areas near İstiklâl Caddesi in Istanbul), getting lost in windy residential alleys (Kotor in Montenegro), narrow Parisian streets, the lost hutongs in Beijing, etc.
I do think working vertically is a solution, whichever way it's approached. Just needs to be a little more organised than areas with urban monorails like Bangkok or Kuala Lumpur where your walking is constantly interrupted with stairs to cross streets, or vistas are blocked by raised train lines.
Toronto has PATH - 29km of underground pedestrian tunnels: http://en.wikipedia.org/wiki/PATH_(Toronto)
I do think working vertically is a solution, whichever way it's approached. Just needs to be a little more organised than areas with urban monorails like Bangkok or Kuala Lumpur where your walking is constantly interrupted with stairs to cross streets, or vistas are blocked by raised train lines.