Even with multiple desktops, you can get clutter in such a way that you don't see the forest for the trees. Actually one of the reasons I don't like / use the whole multiple desktops paradigm is that I find it far easier to keep one desktop organized than half a dozen of them (remember, there's always Command-Option-Click on an app icon in the Dock - there's your multiple desktops right away, since 10.0.0).
So, if you have the need for multiple desktops then you are in that situation probably because you have too much stuff lying around. When you invoke the Spaces thing and see 4 screens in front of you (or 8 in the case of a dual-headed system), you see all the desktops with all their clutter. Good times. You still have the problem of fishing out the one window you are after, either to get to it or to move it to another desktop. And that's the beauty of Expose on Spaces - you can see all 4 of your desktops Expose-ing at the same time, and actually dynamically adapting to the new state as you move windows between desktops. Without Expose, you would just see 4 cluttered desktops instead of one.
Again, I still think that Command-Option-Click is still a far simpler and less confusing than Spaces, but it sure as hell makes a slick demo, plus I have no doubt that there is a large number of people who'll genuinely love it, and all the power to them.
Hope this helps!