someone can explain it better than this but:
traction brackets are used because when u lower your car the diff moves up and so it changes the angle of the trailing arms.. when u lower your car the arms move through parallel (to the ground) and end up sloping down towards the front of the car.. so u get axel tramp. the traction brackets move the mounting point lower to change the incorrect angle of the arm.
u need both upper and lower to be adjustable..
if say u just make the upper adjustable, u can only change the pinion angle in one direction , which may not even help depending on your ride height etc.
ive got a full set of adjustble arms for sale if you want some..
cheers