The Uber article that you referred to as Nate Silver is not actually by him. Its by Ashwath Damodaran (The NYU corp finance prof)
http://fivethirtyeight.com/features/uber-isnt-worth-17-billi...
His basic assumption was to use the taxi market as the total addressable market (TAM)
Bill Gurley, a VC from Benchmark and one of the investors in Uber, lays out alternative scenarios for TAM including going after car ownership. http://abovethecrowd.com/2014/07/11/how-to-miss-by-a-mile-an...
Damodaran's response to Gurley concedes its probable/plausible/possible for uber to get into non-Taxi and larger markets: http://aswathdamodaran.blogspot.in/2014/07/possible-plausibl...
Bottom line: the optionality of uber going into these larger markets is what is driving these valuations