Guido van Rossum writes:
[Michael Sanner] Suppose a new version of a package is released for platform X but not yet for platform Y (maybe platform Y is less popular and the only maintainer is on vacation). ... Without a shared area, each platform can update without affecting the others.
Good argument, and I think I'm convinced by this point; even though duplicating the .py files goes against my grain, that's probably just a sign of my age. But what platform-specific directory should packages be installed into? Is <prefix>/plat-<sys.platform> the right place? Or do we need site-packages/plat-<sys.platform>? (Yet Another default directory added to sys.path -- yuck!)