My instinctive reaction is to say that the two speed measurements are used for slightly different purposes - TAS for airways flying and Mach No for the procedural Oceanic portion of the flight.
TAS would be used in conjunction with the known or forecast wind to provide a Ground Speed for the calculation of ETAs, elapsed time between waypoints and so forth, whereas Mach No is more useful tactically for procedural purposes, for example with successive aircraft on the same track to preserve longitudinal separations. Ensuring that Mach Nos do not differ significantly where aircraft are longitudinally close on the same track will maintain separation quite independently of the actual wind experienced, because all aircraft will be subject to the same headwind or tailwind effect. Also, at the planning stage when entry clearance is initially requested, it would be good practice to clear aircraft with similar planned Mach Nos on the same track in order to maximise overall traffic density.
JD
