There are three kinds of deadlines: fake deadlines, contract/revenue driven deadlines and reality driven deadlines. Most problems with deadlines I've seen come from chronically mislabeling the deadline (e.g. sales don't close, so engineers start ignoring revenue driven deadlines). Fake deadlines sometimes are necessary to get things to some finished state, but they should be used very sparingly. Deadlines should be honest, and when something changes (e.g. some other team is going to be 3 weeks late on their part of the product so being on time doesn't matter) the deadlines should be quickly adjusted. Deadlines are a work-planning and asset allocation constraint, and will cause very bad decisions if they are not understood and managed well.