QSIMDO and interiorpainter, there is more going on here than meets the eye.
First, I agree that some experiments need to be done to validate the efficacy. You have one such in Torker's view that his mill is capable of a lot more than used to be the case. I will also be performing some experiments both in the area of surface finish/performance at various depths of cut, and in the area of measuring deflection and looking for increases in stiffness.
Second, there is a tendency to conclude it's all about mass. This leads one to want to use lead and other materials, or even just sand. But, there is more to it than that. Too much mass will lower the natural frequency of the system, despite the damping, which is considered bad all other things being equal. Reading literature like Bamberg's MIT thesis on machine design, he suggests avoiding sand and lead for that reasons. A higher natural frequency, preferably one much higher, is the desirable outcome.
Third, it's easier to just do it than talk about it. It takes very little effort to do one of these fill jobs. Most of the time involved is waiting for things to set up or planning how to avoid getting the epoxy where it isn't wanted. This was actually one of the few machine shop projects to date where my kids got involved, mixing up Dad's "mudpies", LOL.
The practice, BTW, is common in the commecial machine tool world. You can Google, for example "Hardinge Harcrete" to read about their approach.