Even better, the natural extension (that google maps can't do, not even partially at the moment): allow you to purchase tickets if necessary for the trip, in one go.
EG plan a trip to a different city. Public transit from Hartford, CT to NYC, NY, takes two busses and a train. I should be able to go to google maps, plan the route as normal, click the "public transit" button (that exists), select one of the departure times, click "buy" and have it buy me bus and train tickets for that route. Instead, I have to track down the various means to purchase tickets on several different transit system's websites.