About stylus drag. I am unaware of anyone who has actually measured it and can certainly agree that it seems far fetched. However, we should not underestimate the audibility of timing errors. Digital jitter on the order of tens of picoseconds has been shown to be audible. A pico second is one trillionth of second! At the 44 Khz sampling rate this is less than one part in a billion. It's easy to imagine that 2 grams of tracking force might have a one part in a billion effect even with a 100 pound platter. Measuring such a small effect would be difficult but not impossible.
4yanx, I have never thought of myself or my customers as being supporters of belt drive. It's what we have used and it has and continues to be a very good drive system. I suspect that our customers are not married to a particular methodology and will accept whatever delivers the best sound. But I hasten to add that Teres Audio is NOT abandoning belt drive. The new Teres DD setup will be considerably more expensive to produce and required a much bigger development investment. And while it does deliver considerably better performance it does so at a price. I think it unlikely that we will ever be able to come up with DD system that will compete, at the same price point, with our current belt drive motors. Of course we will try, but don't hold your breath.
So is DD superior? I think that the answer can only be yes and no. When it comes to the best money can buy, then yes I believe that DD has the potential for performance that cannot be matched with either belt or idler drive. But be prepared to spend a lot to get there. When it comes to more reasonably priced turntables then it comes down to the quality of implementation and also personal tastes. As a generality I think that belt drive tends to offer better sound for the money at all but the highest price points.