Why to use "Schottky Diode's"
by Stephen Cook of Spuds Rc World.com

       You should not use a Schottky diode on a reversing ESC, you should use one when you begin to pull high amp’s, say above 30. You can do it at lower amps also but it probably won't help much but it will not hurt.

          Your motor has a coil inside and when the coil is introduced to a switching current from the ESC it generates an inductive load. Remember that your ESC is turning on and off the voltage and current to your motor several thousand times a second. When you charge and discharge the motor the built up current has to go somewhere during the ESC off cycles. Without a diode that somewhere is right back to the ESC’s MOSFET’s, and this can be in the form of a large transient voltage and current spike. This back at ya can cause damage, particularly at high current draws.

          Since your motor is trying its best to get rid of current during the off-cycle, and we don’t want it to go back to the ESC, you install a Schottky diode that will keep the current contained in the motor. By wiring a diode across the motor, we can return the energy back into the motor, instead of the ESC. I believe this returned current also causes the motor to will spin down slower then if there were no diode for improved efficiency.