Once you reach the threshold of isolation sufficient to keep your TT (or CDP) from skipping with light room vibrations, I wonder.I don't have to wonder, I know.
I've tried many rack and isolation systems beneath many phono rigs. A rack affects more than skipping. It also has a major influence on sonics. Preventing skipping is only the minimum level of acceptable performance. It's like riding a bicycle... learning not to fall over is essential, but it's hardly the last word in performance.
A phono playback system is designed to detect extremely tiny physical vibrations, convert them to electrical signal, then amplify the signal by ~8-12,000 times.
Any vibration magnified by thousands becomes audible. This includes both vibrations induced by groove modulations (aka, music) and vibrations coming from all other sources (aka, noise). A phono cartridge can't tell one from the other. It picks them all up equally and the phono system amplifies them all equally.
GI/GO... and a vibration-prone rack sends a lot of GI.