my reaction is that they've over-simplified things at the expense of optimisation of Takeoff Weights.
That is exactly what they have done. We used fixed speeds for all runways, wet or dry, when we first had the Airbus which made life very easy at the expense of RTOW. A sheet was provided with a fixed speed table and set of flex temps vs weight for each runway we used on the scheduled network, the RTOW penalty was not a problem as we only operated short haul from fairly long runways. As the fleet increased in size we operated to more destinations, did longer flights and used some shorter runways the system was dropped in favour of standard optimised runway pages that allowed higher weights and a more optimal flex.