I think you under-emphasize the importance here. It's not just systems, but algorithms, user interfaces, data models, and so on. It's absolutely vital as you work on a large system to have measurable progress toward intermediate goals. Nothing is as bad as the special horror of getting down to the end and discovering that the whole thing just isn't going to work (look at the recent debacle of the Voter News System). I would even go further and state it as a law of nature: no large, complex system can be implemented from scratch, it can only be evolved from a simple system to a complex system in a series of intentional steps.