My understanding is that typical human perception can resolve misalignment between to fixed surfaces, planes, lines, etc if it is greater than 1 degree (imagine a frame builder holding a frame up to the florescent trying to decide if head tube and seat tube misalignment is less than 1 degree). So I can imagine that if the knee is traveling in a plane and a fitter is projecting a vertical plane, then the fitter may be able to make a meaningful judgement as to whether the misalignment of the plane of the knee path is less than a degree or not. If the knee path is not planar then there is no "data" meaningful or otherwise. I should point out that, as far as I know, there is no evidence that illuminates how much weight should be given to achieving a planar and vertical knee path.

With meaningful 3d motion capture data it becomes clear that there is a lot more to knee paths than planarity and verticality.

I also don't do as much with the pedal shoe interface as many because I think there are better ways address knee tracking and having good data has reinforced my thinking on the subject.