I do agree that having separate copies of the data is better, and once we get a second disk array to replace tapes at our third location, I won't care as much about this issue.
However, I do think ReFS makes a stronger case for GFS in primary backups. In the past, it may have been too slow or would consume too much disk space to keep 30 daily, 4 weekly, 12 monthly, and 3 yearly at the primary site. So, we'd keep 30 dailies on primary disk, and the GFS at a secondary site on tape.
With ReFS, keeping the GFS copies on site is very fast and uses very little space. So now it's given me the desire to have GFS at the primary and
the backup site. This way, there's nothing lost if a single backup location goes away. Plus,
our primary backup repo is much faster than our secondary (tape) repo, so for quick restores I'd love to have them also
available on disk. Maybe you mean that this was already possible somehow before ReFS, but for me that was the trigger that made me wish for it.
I understand that you want to encourage customers to keep at least
two copies of backup data, and so it's not currently allowed. But it would be nice if it could at least be enabled through a registry key after some kind of warning or disclaimer.