The main purpose of the D.C. Metro is the same as that of the Bay Area's BART system: to allow people who live in the suburbs to get to work without bringing their cars into the city. Even if you are already in D.C., the metro is only good for riding within downtown or riding from downtown to places that happen to be accessible to a metro stop. Even within the city, Georgetown and the northern neighborhoods are poorly served (probably deliberately so to keep out the "rif-raff").

As for Delhi, things are obviously changing rapidly but when I last visited my friend there in 2008, she had to take a taxi (autos not allowed) from her subdivision in Gurgoan to Dwarka in order to use the system within Delhi. Delhi is so big and sprawling with development spilling far outside the city limits that a truly comprehensive metro system is difficult to imagine.