I set deadline because of the need to get things done, as I realized, most of us like to procrastinate to the last minute, setting myself deadline, it forces we all to push as much and early as possible to get things done right... In many corporate businesses, there are deadline, clientes want you to have it done by such and such day, so you must do it or you lost your client and the payment, etc. So, deadline is good, it's healthy.
However, like corporate businesses, deadline are set within a 'reasonble time period' you don't set a deadline of making 10000 CD by tonight
(bad example, but you get the picture).
The only thing about deadline with filmmaking is sometimes, a filmmaker make an impossible deadline (like 2 weeks before the late deadline for film submission), they want to shoot a feature... that's hard and it will rush you to get the project done right... not to mention, wasting you money for submission fee and then not being accepted and blame everyone else...
Like Will said, the more time you spent on a project, the more expensive it becomes (dollar and sense).
Johnny