ganeshjacharya - 2:49 am on Nov 24, 2012 (gmt 0)
People either click it, bookmark it or select and copy it, neither action requires knowing if it's an underscore or a space.
People would also need to use voice to cite URLs. Suppose I am talking to someone on phone and that person is over a Laptop. How difficult would it be to cite example.com/This_Page_Name_is_What_you_need_to_Open
I deliberately added "T", "P", "W", "O" in capital and other characters in small. In such situations example.com/123 like URLs would be best suited.