WOW no rocks, So these mythical places do exist? We have never had a combine that hasn't had a rock trap. I have seen the damage that is caused by missed rocks in both Rotors and Conventional machines, Not pretty or fun to repair. We demoed a R75 a few years back with just a rock door, and that really us realize how often we pick up rocks, most are just small, but small will still do damage.
If you don't have rocks, probably don't need the trap, if you gottem, you should probably have one