I always thought Functional Specs are more of a burden, at least most of the times. Here’s an interesting take by Linus Torvalds.
A ’spec’ is close to useless. I have _never_ seen a spec that was both big enough to be useful _and_ accurate. And I have seen _lots_ of total crap work that was based on specs. It’s _the_ single worst way to write software, because it by definition means that the software was written to match theory, not reality.
Joel Spolsky has a slightly different take on the matter. And not just in theory.
You can find the debate on slashdot.