It's not that bottom up is good - it's that top down is simply impossible. Try to design a duck this way. It sounds very sensible, and you'll get a sketch in ab

It's not that bottom up is good - it's that top down is simply impossible. Try to design a duck this way. It sounds very sensible, and you'll get a sketch in about two minutes. Not detail it: a couple of hours and 5 pages. Now continue to do this until you no longer have to backtrack: i.e. until what you discover about the duck in the current level of detail does not change at all what you have designed so far. This will have two effects: first, the complexity of the design is equal to the complexity of the duck. That's A LOT of paper. And second, most mistakes in the design will cause the duck to fail spectacularly. On the other hand, with bottom-up design you start with something which eats and reproduces, and improve it constantly until it has a beak and feathers and quacks. It will certainly not look anything like a duck at fist, but it is very likely that from the second or third iteration it will solve 80% of the client's problems. Sometimes even from the first, but that's just luck. It's messy, but it's at least doable.