It could still be useful if the spec is roughly as hard as a simple implementation, in case you have automated methods to find more efficient implementations, guided by the constraints of the spec
Which is still a case of the spec being simpler than the implementation (you are after) ;-)
Very often, the spec is indeed just a very simple implementation. Often you can make the spec especially simple if there are no constraints on the resources it can use, at times even infinite ones.
