[1] Proponents of the waterfall model argue that time spent in designing is a worthwhile investment, with the hope that less time and effort will be spent fixing a bug in the early stages of a software product's lifecycle than when that same bug is found and must be fixed later.
I have consistently saved time and made better products by using BDUF and I’m proud to use it, no matter what the XP fanatics claim.
As we start to build the product, we'll discover a lot of things that won't work exactly as planned.
For substantial projects, the requirements from users need refinement in light of initial deliverables, and the needs of the business evolve at a pace faster than large projects are completed in - making the Big Design outdated by the time the system is completed.
Continuous deployment, automatic updates, and related ideas seek to substantially reduce the cost of defects in production so that they become cheaper to fix at run-time than to plan out at the beginning.
Improving software with the benefit of user feedback is generally less expensive than trying to anticipate and document every aspect of a system with BDUF.