Chasmal et al.:
We mounted a B&K accelerometer on the plinth and/or platter and compare vibration with and without the dust cover while playing different frequencies through the system.
Vibration is much more with the dust cover on. How it translates into better sound depends on your system and your ear. If the listener cannot tell the difference then it should not matter to him/her.
I cannot see how a cover can "block" vibration. Being a hard material, it not only transmits the vibration but also resonates at its own frequency as a result. That should add to the overall vibration, not reduce it.