When your engine provides drive through the driveline (i.e. when you step on the gas pedal), it creates a twisting motion (torque). Since your driveline has friction and mass and all that stuff, it doesn't want to twist right away. So the engine ends up twisting some instead until the driveline gets going. The softer the engine mount, the more the engine twisting will happen (if you've seen youtube vids of engines as people revved them or dyno'd them, and the engine moved a lot... => softer engine mounts). This is power being used to twist your motor, rather than twist your driveline. Wasted power.
If the engine mounts were solid, they would not allow twisting at all, and thus the twisting motion is forced to the driveline. Much less loss of power. But an engine vibrates. If the mounts are solid, this vibration can be transmitted to the car, increasing NVH dramatically, whereas softer mounts would eat up the vibration and smooth it out before the passengers felt it. In a racecar, no ones cares about NVH because racecar. In any other car, people tend to care.
How much power is really wasted with the twisting? Probably negligible. For a racecar, where every bit counts, then it's important. For every other car, it's usually not worth the NVH it brings with it, poly mounts are a much better option.
So, this is a long way to say "What Orange 94 said."