I can see why they offered an incentive program to people who use fuel efficient vehicles, but I think its important to remember that carpooling is a more environmentally responsible practice than just getting a hybrid car.
The goal is to take cars off the road, encourage people to share resources and reduce emissions across the board. Cars do not just pollute when the drive. There is a heavy energy cost that goes into their manufacture. So having 2 or 3 or 6 people ride in one car does much more for the earth than having 6 people drive hybrids separately.