HFS+'s use of NFD also comes from it being an early adopter of Unicode.
When I argued strenuously for n-p/n-i behavior in ZFS (see elsewhere in this thread) I was told that gee, Unicode doesn't describe n-i string comparison the way I was proposing, so we can't do it :( However, fast n-i does follow from the spec, so eventually it got done. I can't take credit for the code, though I did contribute an optimization.
When I argued strenuously for n-p/n-i behavior in ZFS (see elsewhere in this thread) I was told that gee, Unicode doesn't describe n-i string comparison the way I was proposing, so we can't do it :( However, fast n-i does follow from the spec, so eventually it got done. I can't take credit for the code, though I did contribute an optimization.