You are neglecting that the whole system contributes to damping, not only the load bearing members. Taking your assertion to an absurd conclusion, an EG filled metal wind chime would make the same sound as the original. But it doesn't. Once you have met your stiffness and strength goals, adding more of the same metal is a diminishing returns proposition. You might have to add so much material that the machine can't be moved any more.
This is a good if somewhat lengthy read:
http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf
I agree with everything else you said though. Since you mentioned Taig, a bit of trivia: the Taig lathe bed extrusion is concrete filled...
bob