If one rear wheel stopped spinning while you turned it would decrease speed dramatically and you would spin out because its the same as hitting the E-brake in your car, just you can only spin it one way. Also, this would drain your batteries a lot faster and generate more motor heat, and I'm not sure if the motors run 100% as fast in forward and reverse. Also, two motors are unlikely to spin at the same speed, so you will probably end up making permenant left or right hand turns.