I think when you are proposing a fast prototyping phase in programming, you should he able to answer these questions:

1. What don’t we know that we hope to find out with this prototype?
2. Why is this the best approach for finding the answer to that question.
3. When is the prototype going to be deleted?

If you cannot answer to these questions, what you are proposing is not a prototype, it is prototype quality end product. A knock off, if you will. A bad practice distilled.