I'll bet Apple did develop piles internally, but for whatever reason decided that they weren't worthwhile enough to the end user to actually put into a product. (Or, who knows, they'll show up in a later 10.4 build--it's still nearly a year from release.)
It does seem like the whole search (read: half-assed metadata plus content indexing)-based system is a very good idea, and sort of makes piles a moot point anyway. I know a lot of people already who just heap junk into huge folders, or don't bother keeping track of where they sorted anything, and just use the find feature (both on Windows and the MacOS) to find what they want.
Changing the whole internal metaphor of the computer from a filing cabnet and your desk (in which case, piles make sense--"Which heap did I put that document in?) to something more suited to the power of a computer:
Google for your desktop, giving you "What documents do I have about 'fuel cells'?" rather than "Where did I put that paper I wrote two years ago?"
Just makes more sense that way.