Think of it this way when squatting you have to stabilize the weight there is some shifting around. That destabilize feeling is what is actually increases your stability as it is stressing and strengthening all the muscles involved in stabilizing the weight. The machine forces a path and creates stabilization hence you only train the primary muscle not the stabilizing smaller muscles. A strong good knee is a stable knee. I am squatting with moderate weight. Also my knee wraps have magical protective powers bestowed by a shaman.