While metaphors are generally helpful, they are a false crutch to cling to. Metaphors are not required nor are they always the best approach. It is important that the context implied is the one intended and that it matches the desired understandings. Too often, metaphors set the wrong context and help create expectations that are not accurate and which cannot be met.
Metaphors are especially useful when they relate well to a user's or reader's experience.
However, to be used well they must be abandoned when they begin to fail or when they are asked to do more than the limits of their capabilities. A good example is the desktop metaphor used by many personal computers. Recently, a colleague suggested that metaphors could be another way of organizing data. She may be correct, and more thought should be devoted to this question, but I believe it is more accurate to say that metaphors are a means of representing things than a way to organize or present them. For me, metaphors only achieve a cognitive orientation of meaning rather than one of structure.