when the tire hops up and loses traction, the load that was on the engine is no longer there, then when the tire comes back down and regains traction the load is suddenly returned to the power/drivetrain. repeated hops can make the powertrain rock/rotate violently, if it isnt controlled by using poly (or solid) mounts you can very easily break drivetrain or exhaust parts. this is even worse on an AWD vehicle with a transverse engine as now the rocking causes the driveshaft, and thus the rear diff, to move up and down and back and forth as well.
so they are useful, but not very helpful in controlling the wheel hop itself.