About Launch dates, Shows, Demos and planning (Or lack of it)

We've all been there: the big Launch date/show/demo/... comes close, travelers get their travel plans ready, clever ones set meetings in place and plan for what they want to achieve, and developers do the mainstream development stuff.
And then lightning hits the bright manager: I've got to have a demo! it's got to dazzle, sparkle, whistle, pour wine and dance! So what if there's two days to the show?
I've been fortunate to work with several managers in my life. Only few actually set down in advance and planned this stuff: What do you want the impression to be? what are the most important features that are relevant to whom you're about to meet? How much off-mainstream development will this take? are there confidentiality concerns?
After a couple of good runs against time, into crazy hours, I've made myself this optimistic habit: I go and remind the relevant manager twice ahead of time to sit down and think. AND THINK. Nope, that doesn't help.
So I'm left asking myself: what's the story? Why is this so difficult? Why is spending the SAME if not less time in advance, saving efforts, resources, getting a better demo, not worth it?
I don't know the answer, and I'll be happy to learn.
All I can say is that I view PLANNING as a part of QUALITY. if you don't plan, you won't get as good demo/product/meeting/impression as you could have. You'll use more resources than you would have otherwise. And worst: your team will get used to work without planning. Ever heard of the term SPAGHETTI CODE?!
One last point: Some people underestimate the value of planning and executing to a timely deliverable in the eyes of the customer. The time factor is as critical to your credibility as the hours you spent developing and testing your deliverable, double. think of all the resource planning on their end: do you expect them to notify their customers that they're "going to be late"? Would their development staff sit on their hands on their budget?

For those of you out there who are going on meetings, demos, shows, whatever. Do yourselves a huge favor: plan your demos well in advance. the other option's price is too high