Mart,
The 2 rotor speed strategy for the ABC was a product of the fact that the rotor is a big vibration machine, and its stability, as well as the response of the fuselage to its forcing function are very hard to understand and control over a broad continuous speed range. Settling on two Nr settings, and allowing only transient operations between vastly simplifies the problem.
As an example, the entire Tilt Rotor design problem is to create a wing structure stiff enough to separate the rotor and pylon frequencies (especially the alternating tilt mode and the rotor in-plane modes) from the fuselage natural frequencies. Given the types of materials available (the stiffness especially) there are only a small set of geometries (wing box depth, span and chord) that work at all, and none that work if one does not have very fancy computer tools to predict those modes. The V22 is the porduct of that very fancy structural design.
I am sometimes amused at those (Dave?!?) who design a purely aero solution and do not account for the weight and structural impacts of the design decision. This is actually backwards in the real world.