The G-Force crossmember is designed to not use the stock style frame mounting, and mounts metal to metal for added structural rigidity through the center section of the already flimsy chassis.
The stock crossmember uses those rubber insulators to help further reduce noise and vibration in the cabin of the vehicle.
While on a stock vehicle, they do their job to help keep things quiet, in a high performance application, they simply do not have a place, as it becomes one more place for things to move around.