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Originally Posted by CJO
While it is true that you can be pushed backwards while trying to go forewards, using the encoders in a quadrature state requires more processing power (interrupts). If you are only using the encoders during autonomous, it is not overly likely that another robot will be on your end of the field, and the extra processing power may come in handy.
Furthermore, even in other situations, if you try to implement some kind of PID control, the processing required may mean that you will simply need to risk getting pushed backwards.
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there is more to it than that. Usually you use an encoder as the feedback sensor in a PID loop.
With PID loops, you compair what the controller is telling the robot to do, and you use the sensors (encoders) to see what the robot is actually doing. The difference is the error signal that you use to control the voltage to the motors.
For example, if your auton mode tells the bot to go forward, turn right, backup... at some points the controller will be saying (its time to turn) but the bot will still be going forwards. You dont want to mix these signals up - your bot will go open loop (digital insanity!)
Which is why encoders that sense speed and direction were invented. Both are required from the feedback sensors.