Dust covers as generally known and used behave exactly like you stated. The only dust cover I know of that actually has a chance of improving the sound when in use during playback is that from the original Kuzma Stabi and Kuzma Stabi 2 - solid wooden frame and a thick glass on top. That "dust cover" alone weighs about the same as an entry level turntable.
You can make a very simple test to find if your dustcover is actually transmitting airborne vibration - specially as this is head-fi, where people should have headphones and not hate them to death as some do. Place a record on the STATIONARY table and lower the stylus in the groove, at normal listening volume. Listen with headphones. You can tap with finger on the dustcover - what you will hear is not likely to please you. You can extend that test - but now instead of tapping use some music played at normal listening level over loudspeaker and listen for the result on turntable with closed or IEM headphones. That is a bit more difficult to accomodate, but the easiest way is to listen to the phono preamp directly into your headphone amp while playing say CD over speakers - as preamps with full tape loop/dubbing are getting rare these days. That should convince anyone that dust covers act like giant microphones and are specially detrimental with decks without sprung sub chassis, where the vibrations picked up by the dust cover are directly injected into the plinth.
I do use a few records in "never clean it" mode, to test TTs how well they can supress things they never should be there in the first place. Really great analog will be relatively very quiet in the groove, lesser will noise, tick and pop far more. Although great for such purpose, not cleaning records should be avoided whenever possible.
Edited by analogsurviver - 7/16/13 at 12:48am