Godin posted on this, and it struck a chord. In marketing, you are aiming to meet the client's deadlines. Sometimes, you have great clients who can see everything that is involved in something as mundane as sending an email, and they are willing to give it an extra day or quality control. In that scenario, both of you can be on honest time and plan accordingly. (This does not take into account the art director's tantrum or the intern with "food poisoning", but you can work around those thing if you have time.)
What really causes the crisis in most agencies is that only one or two people are on honest time. The rest are on optimistic time, and clients have been taught that the optimistic deadline is correct. So, that is the expectation. Now, what happens when deadlines are hit on optimistic time? As Godin says, someone suffers. It may be the client who has the smallest account, or the one who recently said they were happy with the agency. Maybe the manager with the quiet demeanor is ignored.
Godin makes the point that honest time is the best way to go, but it's not easy. If you could get everyone on your team to change to honest time, and to concede that quality checks take more than 10 minutes, it would definitely be the business changer Godin predicts, but how do you do it?