reusing concrete chunks for new slab?

can anyone think of a reason why one shouldnt reuse urbanite chunks as filler when casting a new slab of the stuff?

Cold joints.

You can use it like giant solid concrete block but not as a filler to reduce the concrete volume.  I am not certain how you would tie all of the reinforcement together.  If you were planning on stacking it with a really cement rich mortar it would be ok but I agree the biggest problem is cold joints.