You are going to have requirements whether you like it or not. It doesn't matter if you discover them during requirements engineering and validate and deconflict them on a simple text document, or if you discover them as you go during coding (possibly after having coded the wrong thing), or if the client discovers them during the "sprint review" of some allegedly agile cult. The only difference is how much money and time are you willing to trade for being called "agile". Ironically the traditional way of doing a requirements stage is the less expensive of all three options. It is also the most aligned with the original agile spirit, since it converges with the client as soon as posible, at the point where changes are the most cheap (changing a line on a text document).
