Cos it doesn't really need to be balanced, your not going to notice an unbalanced rear wheel for road use. How many years did bikes go with unbalanced wheels? No one use to say "oh I wish my wheel had that 2g weight on it"
In my experience, an unbalanced rear only really becomes noticeable after 100mph and if its only a slight misbalance (which it normally is) then it will be 130+, doing those speeds on normal roads, the road is so bumpy that the road surface actually drowns over any misbalance of the wheel.
For a track, if it was a nice smooth home straight then yes probably worth doing.