Some robots have awesome drive with just 2 550 motors, it's all about how you put the power down and how you get it to the motors.
Having 4 motors and gearboxes is just an easy and cheap way of getting 4WD since setting up pulleys, belts and passive axles can be an expensive and time consuming process.
As for 6 motors though I can see it just being a hindrance since it'll create additional unwanted friction when turning.