An extremely important part of project management is the ability to say "no", even if the customer is bringing extra money to the table. Extra requirements have a way of increasing costs in an exponential way and it's very easy to lose sight of your original goal.

Of course this is a problem when you have Congress breathing down your neck and looking for any excuse to cut your program. One big advantage of skunk works projects is that they keep you firewalled off from idea men.