Based on general engineering principles, there are multiple reasons why such a locking device could be unhelpful at best, and dangerous at worst:
- It would be one more system that adds weight and complexity.
- It would take up additional room that isn't available, and making that room would affect the aerodynamics and the weight.
- It would be one more system to check and maintain.
- If it fails, does it fail in a way that causes a running engine to stop, thus creating a dangerous situation, or does it fail in a way that makes it no better than not having it at all?
- Is it more reliable than the jet engine?
- There are situations where windmilling is desirable, turning engaging the brake into one more thing the pilots have to decide to do. Is it worth the cognitive load on the pilots?
Engine failures of any kind are rare, and, as was the case with this incident, often quite survivable. It's not worth the design cost, material cost, weight cost, maintenance cost, or training cost to have a turbine locking device.
In this case, the only thing that a brake would have accomplished is reduction in vibration. With one out of two of the engines down, the pilots would still have brought the airplane in for an early landing, and not continued on to their planned destination.
While this flight was not under US jurisdiction, the Rolls Royce Trent 700 engines used on the aircraft still have to comply with FAA regulations, if for no other reason than that A330 aircraft are flown in the United States. In particular, 14 CFR Part 33 is titled "Airworthiness Standards: Aircraft Engines", and says the following in §33.74 Continued rotation:
If any of the engine main rotating systems continue to rotate after the engine is shutdown for any reason while in flight, and if means to prevent that continued rotation are not provided, then any continued rotation during the maximum period of flight, and in the flight conditions expected to occur with that engine inoperative, may not result in any condition described in §33.75(g)(2)(i) through (vi) of this part.
The points described in §33.75(g)(2)(i) through (vi) are:
(i) Non-containment of high-energy debris;
(ii) Concentration of toxic products in the engine bleed air intended for the cabin sufficient to incapacitate crew or passengers;
(iii) Significant thrust in the opposite direction to that commanded by the pilot;
(iv) Uncontrolled fire;
(v) Failure of the engine mount system leading to inadvertent engine separation;
(vi) Release of the propeller by the engine, if applicable;
Since I can not find any documentation to show that the Trent 700 has a rotor locking device, it must be designed to meet these regulations. Vibration is not listed as one of the hazardous conditions. It is true that vibration can be hazardous, but that's more for a running engine, and vibration monitoring systems exist to shut a malfunctioning engine down.
Other regulatory bodies no doubt have similar requirements. I picked the FAA since it is the one I am most familiar with.