Originally Posted by
MSPeconomist
IIRC there was a German court decision saying that this is not enforceable and flight coupons can be used in any order (although I don't understand how this would work with inflexible tickets). It doesn't seem to apply to the entire EU but might be applicable if your itinerary touches Germany, if you have a German address, or if the tickets were issued in Germany. IANAL.
The court case also gave the airline the right to reprice the passenger's itinerary based on what he flew and the changes he operated. It is only the actual cancellation that was deemed disproportionate.