I should be more clear here. Trying to force a deadline is only part of the problem. You're looking at the deadline as the problem and generally it's not.
Missing a deadline is a symptom of the problem:
1) Changes get made later in the cycle instead earlier.
2) We try to cram too much stuff into a release
The stance that slipping is a feature opposed to just forcing a deadline would be true if people if people were arguing for forcing a deadline but AFAIK no one is. What the discussion is about is how to fix 1 and 2 above so we make the deadline naturally.