The advantage of a Watts linkage over a Panhard rod is the former keeps the axle movement in pretty much a straight line up and down, whereas a Panhard rod allows the axle to describe an arc, ie a curved path.
To my mind, the path described by the axle is only really an issue when the allowable suspension movement is great, ie an off road or rally car but you pay the penalty of carrying more weight and of increased maintenance as there are twice as many joints.
