I'm very flexible. I can do a full split, sit in the lotus position and have no trouble dropping into a full, ass to ankles, squat position. But I can't for the life of me do a full squat with a weighted bar on the back of my neck. I have to bend forward so much so that it becomes a back exercise. A dangerous one at that. Remember, most of a person's height comes from their legs. I'm a tad over 6'3" and when I sit next to my girlfriend who is 5'6" I'm not that much taller than her. Standing, I tower over her. Try it with someone you are much taller than.
So the squat does very little for me as a leg exercise. It's more of a hip and but movement because I have to widen my stance and can only go done maybe parallel at most. Same with the traditional leg press. I can barely break parallel with that machine. Any lower than I start to curl up like a snail and my lower bar loses contact with the pad. Now those leg presses where you are sitting upright is ideal. You can go as deep as you want no problem. Problem is my gym doesn't have kind of equipment. Only squat rack, traditional leg press and hacks. At least with hack I drop into a full squat position and my feet are positioned forward enough so I don't get the shearing effect. I tried this feet postioning while squating using the Smith machine but I just don't like the feel.
BTW, what's wrong with mimic movements done by champions that have been very successful. Jay Cutler has temendous quads. He does hacks and he also does squats. Who is to say that it's the squats and not the hacks that help develop his quads. He also does squats on the smith machine.
BTW, barbell squats bother my knees and lower back far more than Hacks. I've tweaked and injured my lower back many times insisting on doing squats. I've never had an injuries or knee pain using the hack squat. Remember that the best way to push a car is to turn backwards and rest your lower back against the back of the car and push with your legs. You have a lot more power and leverage than if you face the car and push in a running motion. Pushing backwards duplicates the hack movement very closely.