In the book, they don't necessarily argue for an individual private office for every programmer, but argue that a quality work environment has enormous benefits. Interruptions cause huge problems, and random noise can be distracting as well. A door that shuts is one of the most important parts of being able to reduce interruptions and random noise.

A personal office isn't mandatory in order to make progress along those lines. The book actually suggests that you set aside a space for a small group of people (1-4), with furniture that can be easily rearranged.

edit: Just a another comment on the article...

BDUF Big Design Up Front: I think it might be a mistake to assume Joel's arguments in favor of software specifications imply BDUF. A specification is a fluid thing. It helps ease communication between all parties throughout the development process, and lets you save time figuring out some kinds of problems.