I think Gomer Pilot is taking a very sensible, pragmatic view on this issue. Nick, as I see it, the primary issue is not guaranteeing any level of reliability if detailed definitions of limits are kept to - which is clearly impossible - but providing pilots with sensible rules they can work to. Knowing rules (eg, how frequently you can use maximum contingency/take off power) and being happy to apply them means safer, less stressful flying because mental capacity is not taken up with subjective deliberation. It is comforting to know that so long as you operate within the limits you are not unreasonably stressing the aircraft.
The cynic in me would say lack of definition in frequency of using max transient limits provides manufacturers with a grey area to argue against any litigation following failure.
I believe frequency limits should be specified, or at least recommended. Depending on the environment and the operation, some acft will be less stressed by full exploitation than others, but that's the same with anything, such as max continuous power. As pilots we all know we can influence longevity of components and reliability by determing how close to limits we operate to, but at least we want to know what these are, and only the manufacturer can tell us that.