[The common definition of estimate is] "the most optimistic prediction that has a non-zero probability of coming true." ... Accepting this definition leads irrevocably toward a method called what's-the-earliest-date-by-which-you-can't-prove-you-won't-be-finished estimating.—Tom DeMarco

What you give up with approaches that leave requirements undefined until the beginning of each iteration is long-range predictability about the combination of cost, schedule, and features you'll deliver several iterations down the road. As Chapter 3, "Value of Accurate Estimates," discussed, your business might prioritize that flexibility highly, or it might prefer that your projects provide more predictability.

...found in the promising Software Estimation: Demystifying the Black Art

John E. Searle <a href=books.google.de>"Speech Acts: An Essay in the Philosophy of Language" and an elephant never forgets his promise.