Processor cycles aren't like gold that you can save up. Your machine is constantly "spending" them whether you like it or not. That's one of the inherent drawbacks of living with forward-moving time
Yes, the "gratuitous" animations do use these precious processor cycles, but their huge usage numbers in top are because nothing else is using the CPU.
Here's a test. Do things that stress and contest for the CPU cycles. Play several quicktime movies. Do a large update on a database. Hork up the Classic environment with some demanding task in a Microsoft app. Do them all together. And then start minimizing and maximizing windows.
Does the window manager steal cycles from these other tasks? Certainly. But you'll also notice that the genie effect drops frames and becomes much less smooth under these conditions. Most of the beauty and slickness comes from what would mostly be idle cycles. There's also the argument that since your attention is squarely focused on a particular window when you're diddling with it that it SHOULD have more of a priority than some of the other windows in the system.
When your machine is mostly idle and you see the window manager sucking up huge amounts of processing time in top, that's because it can. The machine has no higher processes demanding cycles. It'll take the extra cycles and make a really eye-popping effect.
My conditioning upon seeing eye-popping effects like these was that they were a performance hit. But I was much less bothered in seeing how the system DID drop frames and not insist on "perfect" animation when under stress. Had it not then I would have been very bothered by the animations. If you find them unattractive or disconcerting then that's one thing, but I don't find the "wasteful" arguement very convincing the way it has been implemented.
Of course there are many traditional Mac users who are accustomed to the machine stopping when you opened a menu, so to them they would be more disconcerted when they see the frames of the genie effect dropping than noticing a little slowdown in their database update when they start interacting with the user interface.
It's still much better than the Classic Mac OS and in my opinion better in some ways than X11 in handling the balance between system performance and responsiveness for the user.