Pretty sure how the knee joint works is that as one goes deeper into a squat the more surface area is covered and attually the more stable the joint. Also the even pull of the hams and quads stabilizes it.

On machines, like leg presses and stuff, the load is unbalanced and causes and imbalance i the recruitment of the hams/quads-that'll mess up your knees.

Or using a Smith machine. I see people trying to keep thier knees directly over thier shins and they are leaning way back into the bar... Well the design in the body is one to squat, but how the hell do they expect to pick up a suit case, buttun up thier kids coat, take shit in real life?

Think about how you get in a car, most people do a one legged squat.

Squatting is important!!!!!!