> Heavy sprints are important to achieving milestones for major events
No, they are not. Heavy sprints reflect planning.
If we're talking about a one-off, once-in-the-lifetime-of-a-company type of event, it's understandable.
But in most situations like this, one can usually chalk it up to "lack of preparation on your part does not constitute an emergency on my part". Whether you agree or disagree, the net effect is that it kills morale. Absolutely kills it, as the product development team loses faith in the planning powers-that-be.
Heavy sprints are most often a reflection of upstream planning, and a poor one at that.