[Barnsnake]

Constant velocity (CV) joints are so named because the rotational velocity is maintained when the joint is operating through an angle. This allows for a single joint, or a pair with differening angles.

U-joints, when operated through an angle, result in an output rotational speed that is accelerating and decelerating twice per revolution. In order to use such a device in a driveline, you must use a pair of dimensionally identical joints, aligned in phase, and with identical, but opposite angles.

In the case of a vehicle driveshaft, here is the exaggerated version of what happens (for this example we have large u-joint angles):
Say the tailshaft of the transmission is spinning at a constant 1,000 rpm. The rotation is transmitted to the u-joint yoke, and thus to the driveshaft. The main section of the driveshaft is now varying its rotational speed from 950 rpm to 1050 rpm twice per revolution. (Charted as time versus RPM it would describe a sine wave.) This irregular rotation is now transmitted to the rear u-joint, which is precisely aligned with the first u-joint, with an equal, but opposite angle. The u-joint translates the irregular rotation to the differential pinion as a constant 1,000 rpm.

This is why pinion angle is such an important setting. If the angles are not equal, the total cancellation doesn't happen. The net result is that the engine/transmission or the rear axle must accelerate and decelerate twice per driveshaft revolution as the car moves down the road. This causes an irritating, or potentially destructive vibration.

As a side note, the acceleration and deceleration of the driveshaft consumes some power. The heavier the driveshaft and the greater the angle, the more power is consumed.

The CV joint avoids this phenomenon by sliding the rotational axes within the joint. This sliding action is also an energy consumer through friction.

Why use u-joints at all? They're cheaper to make and can be exposed to all manner of elements. The CV joint must be precisely machined and sealed with a flexible boot.