Those folks who are fortunate enough to have their home directory on a netapp filer
have for years been able to "cd ~/.snapshot/" and find a special directory of
historical versions of their files. These are stored efficiently because of the nature of
the WAFL filesystem. With reflink, it would be possible to create a lightweight
version of historical snapshotting: you'd have a daemon run every night (for
instance) and recursively reflink the current state of all your files into a directory
tree at ~/.old-versions/<date>/ - then, if you ever needed to go back to an old
version of a file you could just look in there.
With reflinks this would be very fast and would not use up loads of disk space
(though there would still be quota concerns). It would make time-machine or Netapp
.snapshot-like functionality easy to implement efficiently on single disk systems.
Probably the most quoted reference for stuff like this is the Elephant research
filesystem, about which there are a number of decent research papers.
Another use that I've seen mentioned is the ability to make checkouts / clones in
moderen version control systems go faster and be more lightweight in terms of disk
storage - for instance, cloning a git repository could transparently share all the
underlying data (including the working directory!) using reflinks. Similar tricks being
possible for the other VCSes.
Finally, you could probably have a daemon that rummages around the system, finds
identical files and unifies them on disk using reflink in order to save space.