Even more, software development now involves an assumption of the average manager or customer that some extra feature can be added halfway through.
Is the customer going to be happy with "we've billed $X for a formal spec and that means that we can't make the change that you want, that seems simple without $Y dollars for changes to it and the code." Notice that software methodologies have gone the opposite direction here, with Extreme Programming basically aiming to make all of the programmer's activities revolve around exactly what and only what the customer has actually requested.
But the features are not arbitrary. The vast majority of features are common among many applications, and a template formal spec can be built to satisfy those features. Once formal spec of the building blocks is created, there will only be the small portion of unique code that needs a unique formal spec.
Is the customer going to be happy with "we've billed $X for a formal spec and that means that we can't make the change that you want, that seems simple without $Y dollars for changes to it and the code." Notice that software methodologies have gone the opposite direction here, with Extreme Programming basically aiming to make all of the programmer's activities revolve around exactly what and only what the customer has actually requested.