[FRiSson]

Simple. The car is designed with as much weight towards the center as possible. That means that the car is designed to essentially pivot around its center. If you place weight towards the ends of the car, you disrupt that carefully engineered principal. You will then create forces that will tend to swing the car away from its center axis, much like placing a weight on the edge of a spinning top will cause it to swing off-axis.

The only time weight might help is if you are stuck or getting little traction up a hill. But once you start taking turns, the weight will be a detriment to the control of your car. So if you must, keep the weight near the center of the car (back seat or passenger foot well) then move it to the trunk if you are stuck.

The idea of putting weight in the trunk comes from your grandfather's day when Detroit cars were too light over the rear axle. Those cars were much larger and differently constructed than the FR-Z.