File descriptors aren't simply a "resource" in the sense that memory is. Closing a descriptor may have significance beyond the process which closes it. If it refers to the write end of a pipe or socket, closing it may cause the reader to receive EOF; if it refers to a file, any locks will be released; and so on.
Yes, I think this is the crux of the matter: closing a file-descriptor is not just a question of releasing resources, but it has actual side-effects visible by the outside world (e.g. closing a pipe, or actual write-back if the file is on NFS, ...) so it is important for programs to close FDs explicitly when it needs to happen, rather than leave it to the GC to do it whenever it feels like (which can be much much later if you get unlucky with your generations). Stefan