well it's a lazy reason really but I don't want to take the one off my current AV10 motor, it's the original one and I don't want to mess with the case bolts again. the other motor I built (stock av10 cases) doesn't have one and if / when I swap it out I plan on using the same spring which is attached to the 51v frame (not a doppler red spring).
long answer... if that makes any sense