Originally Posted by Rick Alston
I've got to believe that anytime you dampen movement between the trailer and the tow vehicle, both the trailer and the tow vehicle are better off from a vibration perspective. If that is true, then the vibration (energy) created by the trailer hopping about is absorbed by the hitch because it is not felt in the tow vehicle.
Over59 has a valid observation about bad axles. The air hitch isolates the driver and tow vehicle from vibration caused by the trailer. If that permits the driver to feel comfortable driving at higher speed, it may make the vibration in the trailer worse.
Each vehicle has a resonant frequency determined by its suspension and wheelbase. Think of them as giant tuning forks. If you have a rigid connection between the tow vehicle and the trailer, you have two tuning forks connected at the handles, and the vibration of one vehicle can influence the other.
If you insert a rubber pad between the two tuning forks, you isolate the two vehicles and allow them to vibrate closer to their natural frequency. This may reduce harmful vibration, assuming that the vehicle suspension was well designed and loaded. In the case of some trailers, this may not be a valid assumption.