When you're driving hard, there are forces exerted in all kinds of different places. Not only are there the G-forces you feel during cornering braking and acceleration, but there are the forces that the car exerts on itself.

The more power your motor is making, the more force it is exerting on it's mounts. Remember that whole "for every action there is an equal reaction" thing? This transmission brace is designed to help keep the motor and transmission unit generally in one place while driving. It adds a fair amount of NVH (Noise, Vibration and Harshness) to your car. Anytime you stiffen a mount for something this will happen. Generally this NVH is a much smaller problem to the consquences of a motor/transmission moving around a lot in the car. Ever try a 3rd to 2nd downshift while cornering hard? Coupling the cornering G-forces with the movement of the motor when you blip the throttle, the whole unit moves quite a bit. This makes the geometry of the shift linkage change to a point where a smooth shift is almost impossible. You may even get the wrong gear.

So keep the reigns tight on that motor and install one of these braces.