Hi Hugo,
Good point, and one certainly worth bearing in mind. However, in terms of your last sentence, I don't know about it being done in "arbitrary fashion", or the concept being "simplistic". It's simply about reporting, in a common sense and practical way what you hear, based on your [in this case, in reference to myself] years of experience comparing various belt-driven turntables to their direct-driven counterparts.
And given the use of recordings that don't display any inherently audible instability, which after all was the only way you knew that such was present to measure, I'd still contend that any well-designed direct-drive turntable would outperform virtually any belt-drive design *in the specific area* of pitch/speed stability, notwithstanding the possibility that the latter could, overall, still sound better, or rather the sound produced be more preferable to the listener.
Since Andrew seems sensitive to the effect of speed/pitch instability, it therefore seems sensible that he should target turntables which excel in that area, and from having achieved that platform of accuracy, then 'tune' the sound, in terms of musicality, to achieve the desirable results, through judicious selection of partnering ancillaries, such as arm and cartridge.
That's simply what I would do in his position; not some definitive guide that everyone should follow
Marco.