The Watts doen't stop the deflection what it does is keep the hub upright in the event that the axle beaks..because the axle is the upper control arm.
The radius arms keep the lower control arm from flexing under acceleration. The lower arm is a round pipe that can flex!
Without the radius arm, the rear wheel "shudders" from front to back,
With the radius arm it can't deflect at all....the car squatts and takes off!!!
The reason also that they need to face rearwards is that the metal tube that you use to manufacture these rods is stronger in tension than compression.
John