Deadlines are a way to get people to finish what they're committed to. People (at least the procrastinators in us) have a tendency to delay or to meander away from what they should be focused on and a looming deadline causes them to refocus.
As others mentioned, it's not really an incentive.
I've been experimenting with Scrum (an agile methodology that should work for non-software projects as well). There are a number of components that replace / augment deadlines:
- Break down the work into 1 and 2-day increments (so it's simpler to judge progress and know what's expected of you). This is still a deadline, albeit a shorter one.
- Report status daily to the team. If this is a team effort, the psychological effect makes you focus on what you're doing so as not to let the team down. Note that in scrum, teams sign up for shared goals.
- Iterations are 30 days long. That's another deadline, though the fact that it comes every 30 days and all the work needs to be completed by then has a strong effect on the team.