Yet mdf is used for by far the most commonly used material for cabinets, and, let's be honest, the guys a TNT are hardly techos. If you look at the BBC papers on different materials, you'll see they went into the subject in great detail http://downloads.bbc.co.uk/rd/pubs/reports/1977-03.pdf
. Convinced me enough to make my own speakers out of ply, but, frankly, next time I'll use mdf because it is so much easier to work with.
You should also consider the application of the material. In the case of turntables, you do not want materials that resonate or cause resonances within the audible spectrum, because that produces coloration. In fact, you want something pretty much acoustically dead if you're going solid plinth, so ply is not a particularly good choice, especially when bearing in mind the fact the different woods used in different plies will each produce a different effect. In other words, Beech ply is going to have different properties to Okoumi, for example. If, as you claim, people making their own turntables are using ply, then they're basically using the most hit and miss method out there. Mind you, it could also be they like the sound of the coloration as many of us do.
MDF, on the other hand, and for the reasons you cite, is eminently suitable as the basis for a turntable. You want something that is as acoustically dead as possibly, allowing the cartridge to do it's work in the cleanest possible fashion. That's why many manufacturers use MDF or variants thereof in their turntables. I have no idea why anyone would want to use laminated bamboo or slate in their turntables.
Your last comment makes no sense.