Varying the speed in the descent is the most efficient way of managing the descent profile. The Airbus is pretty clever and knows this. If the aircraft is allowed to make a managed descent it uses a variable speed band in order to achieve the correct profile thus reducing the requirement to use highly inefficient speed brakes. This ruling has taken away a huge amount of flexibility, could actually increase fuel burn in some cases and will increase R/T loading as pilots will have to keep asking if they can fly at higher speeds in order to make the numerous altitude constraints.
I think the 250kt below FL100 ruling is a great idea but to remove the ability of the pilot and aircraft to efficiently manage the descent profile for the entire descent phase is, quite frankly, a step too far and may actually defeat the original objective of saving fuel.
Surely better for the airlines to tell ATC that most aircraft these days are descending at about 270 kt anyway due to the low Cost index, but allow us the flexibility to alter this, for instance, if we have a late descent clearance and can't make an altitude constraint. If ATC need a fixed speed for separation and streaming then we’ll do it. Common sense please!