Absolutely agree with you about needing a well defined scope and how that can reduce the chance of an overrun. Likewise, laying the hammer down simply because

Absolutely agree with you about needing a well defined scope and how that can reduce the chance of an overrun. Likewise, laying the hammer down simply because an arbitrary deadline is on the calendar isn't good project management. But I still argue that a good, reasonable deadline that everyone on the team has a say in setting acts as a commitment -- the team is saying, "yes, we agree to deliver a product in this timeframe." Once you've made that commitment, the deadline then acts as a forcing factor that drives everyone to prioritize the most important tasks.