Why Use Aluminum for a TT Platter?

In my experience aluminum is not the best platter material. But there are good reasons that it is common and popular. It is easy to machine, reasonably inexpensive and when layered with another material or at least a good mat it performs quite well. Brass is a far better sounding platter material than aluminum, but costs 4 to 6x as much as aluminum. So as good as brass is, it's not a great value. I find that paper based phenolic, like Garolite XX, makes an exceptionally good sounding platter.

The discussion about vibration from motors is missing the most important point. It is not vibration that degrades sound but cogging. Motors with a lot of cogging usually, but not always have detectable vibration. Because of the isolation afforded by the belt, most belt drive motors have a lot of cogging. Because DD motors have no isolation they must be designed to have dramatically less cogging or they would sound terrible. Cheap DD implementations often have mediocre motors that have too much cogging (still far less than belt drive motors) and hence sound ragged. It's not that DD is bad but it's that a poor implementation of any sort will sound bad.

I find that DD has the potential for goodness that cannot be matched by even the best belt drive implementations, but bring your wallet because it is both expensive and difficult to get right.

Livemusic, Yes Cocobolo is still the best platter material I have heard, but Garolite sounds nearly identical and is far less expensive.

Tbone, Coreless, slotless motors have very low cogging, but it is not zero. A top quality DD table is remarkably sensitive to even extraordinarily small amounts of cogging. I have no first hand experience with the fine motors you cite, but suspect that the very low cogging they exhibit is still audible.

Higher platter mass generally translates into better sound. But as is often the case with audio there is no consensus. Heavy platters sound different than light platters. Most but not all prefer the sound of heavier platters. I have experimented with platters up to 75 lbs. For my tastes I found that heavier was always better and never found a point of diminishing returns. But anything beyond 75 lbs starts getting very impractical.

An interesting discovery was that both belt drive and DD implementations seemed to benefit from heavier platters. But with DD the benefit from a heavier platter was significantly less than with belt drive.

Cogging is measured as torque ripple. It is the amount of variation in the developed torque as the motor rotates. A single phase AC motor by definition has 100% torque ripple since at the zero crossing of the waveform no toque is generated. DC motors have much more constant torque and typically will have torque ripple values of 10 to 15%. The motors used in DD tables have multiple overlapping phases that dramatically reduce torque ripple. In theory a three phase motor will have less than 1% torque ripple. But that is only if they are perfectly constructed and driven with perfect waveforms. But even a crappy three phase motor will have relatively low cogging compared to a single phase AC or DC motor.

Typical motors will have windings wrapped around a laminated iron core. This focuses the magnetic field and makes the motor more efficient. Core-less motors (AC or DC) have copper windings that are formed without a core. They have less cogging because there is no attraction between the magnets in and the iron cores. Cogging can also be reduced by angling the cores so that they have equal attraction between the magnets and the cores as the motor is rotated. You can tell how much the cores affect cogging by turning the motor by hand without power. Core-less motors are less powerful, more expensive and usually have less cogging.