Yes, the springs sag due to age. This lowers the car slightly too. The springs support the weight of the car, the struts dampen the movement.
No cons that I can think of other than cost. I would also replace the front suspension bushings: http://www.myturbodiesel.com/1000q/a4/LCA-bushing-Jetta.htm The front doesn't wear out much but the rear does, quite fast because it's soft.
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