alt Hacker NewsAgile largely came about because we thought about where we wanted the product to go, and the steps to get there, and started building, and then it turned out that the way we thought we wanted to go was wrong, and all of that planning we did was completely wasted.
If you work in an environment where you definitely do know where you want the product to go, and the customer doesn't change their mind once they've seen the first working bits, then great. But I've never worked in that kind of environment.
It helps to at least write down requirements. And not requirements in that "it must use Reddis", but customer, user, performance, cost, etc requirements.
A one page requirements document is like pulling teeth apparently.