Frame geometry is the crux of it but not all of it. Acceleration comes to mind. Because of the bigger wheel size, adjustments to the geometry to compensate for the bigger wheel. Does that affect other aspects of handling/geometry design? For example, do they open up the rake on the 29'ers to increase handling for the bigger wheel? And if they do, a wider rake means slower response time, does it not? So if compromises in performance have to be made to accomodate the bigger wheel, would not the 'tweener wheel size be better?
No matter the frame designs or geometries, can one argue against this logic : The 'tweener is better than the 26 for all the reasons that a 29'er is currently better than a 26, but the 'tweener is also better than a 29'er for all the reasons that a 26 is currently better than a 29'er.