It's all well and good to fly in the area and take the risk once. Multiple sectors in contaminated areas may prove more complicated due to accumulated damage.
The only way to assess the damage after the flights is to perform borescope inspections on each of the engines, which accounting for cool down time would take 4-7 hours per engine and is obviously not practical after every single flight in these conditions. The first inspections may reveal no damage but after that it's russian roulette, the test flights may prove good
PR exercises but in reality the information gathered would be too little to make any kind of accurate assessment based on each type of operation.
Our safety environment is based on reliability (historical data) and preventative actions. We do not have enough data to categorically state that the ash will not bring down any aircraft or significantly damage one so we are left with prevention.
If even one aircraft of the 20000 that operate daily in Europe has an incident or God forbid an accident, the risk would be completely unacceptable and it would not be long before the public lost faith in the airlines. The backlash on the authorities I'm sure would also be something to behold.
Safe operation in ash has been demonstrated numerous times and confirmed by evidence of erosion on radomes and engine inlets. Unsafe operation of aircraft in volcanic ash has also been demonstrated numerous time >50 via symptoms of EGT and or stall as well as buildup of ash in the aircraft bleeds and sensors.
I would suggest therfore that evaluation of symptoms/evidences after a flight provide room for risk assessment and a way to operate within practical everyday risk levels.
However, I think it prudent not to push the boudaries of visible airborne evidences and attempt to find the boundary of where it`s unsafe.