A relevant column in ACM's Interactions magazine by Lars Erik Holmquist about the use of mock-ups and prototypes in interaction design. His claim is that it is certainly fruitful in participatory design (where users are brought in very early in the design phase) but "there is a danger with putting too much faith in what is, after all, only a shadow of the real thing". He clarifies his point using the cargo-cult metaphor

What is the difference between the positive and negative uses of representations? (...) cago-cult is a certain form of religious movements started to spring up in the Melanesian islands in the South Pacific. These religions thought that the goods—the cargo—that started to arrive on ships and planes had a divine origin, or more specifically that it came from their ancestors.(...) The Melanesians reasoned that if they could build exact replicas of the white man's artifacts, they would receive the same benefits. What they failed to realize was of course that their replicas, made from bamboo and straw, while superficially similar to the real thing did not capture the essence of the original artifacts.(...) We can define cargo cult design as creating a representation without sufficient knowledge of how it actually would work, or presenting the representation while not acknowledging such knowledge.

Then there is a nice discussion of the concept of representations:

In a design process, representations are a physical embodiment of something that otherwise would only exist as an abstraction. Without getting deep into the epistemological definition, we can say they are the embodiment of knowledge. But mock-ups and prototypes represent knowledge in different ways.(...) However, to give any kind of reliable information, the representation must give a realistic impression of the intended end product. If the representation is based on insufficient knowledge of real-world factors, presenting it to potential customers or testing it with prospective users will not make much sense

He concludes about to use mock-ups and prototypes:

When presenting a mock-up or prototype, the interaction designer should always ask:

Am I fooling myself? Do I really have enough knowledge of the technology and the users to gain valuable insight from this representation, and will it help me to construct the "real thing"?

Am I fooling the layman? Is there a risk that people mistake the representation for the real thing, and thus believe that I have solved problems that I have not?

But the interaction designer should also see the value in representations as generators. Even when the knowledge that goes into a representation seems questionable or even irrelevant, it can still be valuable, as long as the results are treated responsibly. There is value in toying with and the possibilities of technology and being inspired by them; prototypes that may not seem useful can give rise to many unexpected ideas and eventually form the basis of successful products. With the concept of generators comes an explorative attitude to the development of interactive artifacts. Interaction designers should be encouraged to take representations, prototypes and mock-ups of all kinds as starting points for exploration—but never accept them at face value.