When The Joel Test
first appeared, one of the biggest sore points readers reported had to
do with writing specs. It seems that specs are like flossing: everybody
knows they should be writing them, but nobody does.

Why won't people write specs? People claim that it's because they're
saving time by skipping the spec-writing phase. They act as if
spec-writing was a luxury reserved for NASA space shuttle engineers, or
people who work for giant, established insurance companies. Balderdash.
First of all, failing to write a spec is the single biggest unnecessary risk
you take in a software project. It's as stupid as setting off to cross
the Mojave desert with just the clothes on your back, hoping to "wing
it."

Read more here:

http://www.joelonsoftware.com/articles/fog0000000036.html

P.S: