It would probably be exaggerating to say that one could walk to the terminals in the time it takes the AirTrain, but not by a lot. And that's even when they don't make random stops not at the stations.
What is the point of having two sides to the station when you only use one and the other train must wait? What is the point of having two rails if one car must stop as the other passes? Why were the people who came up with this on drugs?