Shake a CD player without a buffer. What happens? That's right. The disc skips. We also know from people's reports on the R-4 that vibrations do effect the 1s and 0s. With the CD player, there's oversampling and error correction, but it seems reasonable that when there's errors that have to be corrected the sound degrades from when the disc is beng read without errors (or that's the theory anyhow -- I do not have independent knowledge)
And don't forget that CD players (as most people use them) have analogue output stages.
I had the famed and subsequently defamed Optimus 3400 running in my system when they were in vogue around '93. Vibration damping DID make a difference to the sound of that lightweight plasticy box. I also followed the prevailing advice and got an external D-A converter for it, and of course had to buy a digital cable. And then an X-10D (i think it was called) analogue tube buffer, which required extra analogue cables. Each of these things noticeably improved the sound as promised. I also put those clip on things on all of the cabling but didn't hear the difference Sam Telig promised. I passed on getting the custom DC power supply and NOS tube.
Then I sold the three boxes and, for less than all of that stuff had cost, got a Sony ES series CD player that sounded as good or better (a questionr of individual taste), and also had a remote, more functions, was built like a tank, didn't require extra damping, and replaced a mess of cables, power cords, tweeks, and extra boxes. Wish I upgraded to it in the first place instead of screwing around with tweaks.
As I wrote earlier, I agree that there is a ton of BS out there about tweeks. Some do work. But they are not necessarily the most cost-efficient way of upgrade your sound, especially when you factor in the money you throw away trying different stuff before you find the ones that work.