With the UI, it's a different story...

[...]
When designing UIs like websites, continuous changes of the user interface due to fast iterative design may conflict with user expectations and learnability, provoke inconsistency and possibly lead to user dissatisfaction. Evaluation of small releases with stakeholder participation does not ensure that the whole system provides a consistent conceptual, navigational or content model.
[...]
Increasingly, software engineering (SE) and UID have to cope with a shorter time-to-market, whereas the quality of the delivered software must not suffer. This continuous shortening of development lifecycles is a great challenge to both project management and the applied methods and tools. Therefore, many usability engineering (UE), UID and agile method experts have developed light-weight approaches or so-called agile usability engineering (AUE).