It depends on the lifters build:
In a close (bodybuilder type squat) that is true. But in the more common powerlifting stances( moderate to wide ) the hams, glutes and adductors become more the primary movers ;especially as the stance moves out. The quads are still very much envolved but not as much as in the bodybuilder's close stance squat.
They're just as important to a squat:
depends some on body type also I think guys with long legs and short torsos might squat more upright which would seem to be more of a quad squat short legs/long torso gives a lot of forward lean which would seem to bring in more hams/glute it also has to do with where you hold the bar on your back.....holding it high on the traps is more of a bb squat and is more thigh oriented in any case you cant totally isolate either quads or hams etc
I don't care what type of squat I'm doing(wide, close, medium stance), although I do feel them in my hamstrings I also feel it in my quads just as much on all of them. In my training I concentrate on quads just as much as hamstrings. Just ask Ed Coan are quads important in the squat, or any olympic weightlifter for that matter.
Quads are secondary muscles in the squat:
I've heard people go so far as to say the you really don't use your quads that much in the squat, which is just silly when you think about it.
They're secondary although still important:
There is alot of merit to saying that squatting uses the hams and glutes as primary movers. The wider the stance is the more the adductors become movers and not just stabilizers. The squat is a hip and pelvis motion and not necessarily knee extension motion. Yes the quads are used to help straighten the knee, but in my opinion they work in more of a stabilizer fashion.
The quads are more of stablizer to a wide stance squatter, but from personal experiance I can tell you if you neglect them your progression will be greatly hindered.
Maybe the quads are the dominant muscle:
You MUST have quads to walk the weight out, stabilize the weight, and they are definetly part of the knee extension in the movement.
Didn't Kaz say something about how the primary movers in the squat were the quads, and he didn't know why everyboy neglected them.