The point of that was that, at the time, GHC had changed FilePath in a way that was in fact rather broken.  It has since been fixed, but still has no way to represent a *raw* FilePath but instead makes the non-POSIX (and in fact, only true on OS X and even then only guaranteed for paths used from Carbon/Cocoa apps) assumption that FilePath-s have known encodings.

The ByteString version of the module is intended to allow apps that actually work reliably on Unix/POSIX systems to be written.  The other use would perhaps be nice, but is already covered by the ByteString package, and while I understand that it is not considered appropriate to want to be able to write reliable filesystem-using applications on POSIX systems in most developer communities these days, some of us would still like the ability to do so.