Yes, they are heavy. They are not meant to go fast, just to provide an alternative to walking, taxis, and Metro/bus. Heck, my 1985 Stumpjumper weighs almost 40 pounds, and it was a near top of the line racing mtn. bike in its day. It's relative.

The Capital Bike Share bikes (and I would assume the NYC bikes) sure are faster than walking. They are aimed at people who don't have bikes handy at work. People use them to go to meetings in various places downtown. They really do increase the options for getting around downtown.

In other cities, these bikes tend to gather at the bottom of hills because people don't want to ride them up. I doubt that lighter bikes would change that pattern, though.