No two motors will ever turn with the same angular velocity given the same voltage. If you power each of your Rover 5 motors with 12V (I don't know what they're rated for), you'll see that each motor will spin at slightly different speeds. If you want to guarantee you're traveling in a straight line, you need to implement velocity control on both wheels. One method of doing this is implementing a PID controller on the drive wheels to ensure their velocity is the same, based on encoder ticks per unit time. Otherwise (let's assume you have two wheels that are driving the vehicle) one wheel will turn faster than the other, and you'll slowly drift in one direction.
However, you may want to turn a vehicle with no steering control! That means you want to turn your drive wheels at different velocities (albeit this will cause your wheels to slip horizontally and thus cause you to lose some traction/power), and so you need two different encoders that will be the input to two different velocity controllers. For a tank like system, if the front left wheel encoder is ticking and the rear left wheel encoder is NOT ticking, then maybe your tread has fallen off! It's really very useful to create a robust system.
Edit: Man I keep editing and adding more stuff...having multiple encoders will also allow you to identify failures. If you see that one wheel has stopped moving, it could be stuck on something and/or broken! This could allow you to halt the system and tell the operator that a mechanical failure has occurred with, for example, the front left wheel. This can only be done if you have more than one encoder.
As a side note, it's always good to have a redundant system in case one breaks!