We asked our three questions on deadlines of Jon Moore, Technical Fellow and Chief Engineer at Comcast Interactive Media:
Q1: Do deadlines empower your teams?
I've found that deadlines can cut both ways. On the one hand, an achievable but challenging deadline can really get teams to focus; on the other, a seemingly impossible deadline can crush morale. Having a fixed deadline with variable scope (where the minimum viable product is achievable by the deadline) is a perfectly fine timeboxing mechanism that can line up well with longer lead-time activities that must be coordinated around a big launch--for example, marketing efforts.
Q2: Do your customers expect delivery on deadlines?
Yes, internal customers often expect delivery by negotiated deadlines. In a company with many teams, estimates like ideal hours or story points do not carry well from team to team, but calendar time does. Asking when another team can deliver a dependency serves as a useful unit of exchange.
 I have even heard of teams from a different company that estimate in kilograms!
Q3: What are you doing to meet customer expectations (in response to or in lieu of deadlines)?
I've seen have success with tried-and-true mechanisms from Scrum: prepare all the user stories, estimate in story points, then project out based on historical velocity. If the scope/date line doesn't pan out well (or is close), manage expectations with the customer as early as possible to discuss scope/date tradeoffs or risk.