The challenge with the free/common courtesy model is that casual carpools are just not getting enough cars off the road. Lyft is solving the two biggest challenges I see in carpooling 1) convenience of having the app route you to people who are close enough to your route and can be dropped off without adding too much time and 2) making you feel more comfortable picking up strangers. Adding money to the equation to encourage more drivers is fine with me as it reduces car traffic and pollution.