Many developers can talk about an 'agile' approach where the customer is free to make changes throughout the life of the project. However, if the software is not created with sound principles of modularity, change will still be costly.

When using a modular system like Drupal properly, building working prototypes of features can be quicker than hours of discussion and analysis; so even if the initial instinct is wrong, we've lost nothing by throwing away the prototype and starting again.

However, we can almost always re-use parts of the prototype, so a Prototype-then-Analyze-and-Adapt ("Agile") process is superior in every way to a Plan-then-Build ("Waterfall") process.