A common deployment strategy might involve creating arbitrarily named
deployment target folders, installing a new flow instance, warming up
caches and either switching symlinks or web server web root settings
to that new directory.
Using %FLOW_PATH_ROOT% as default supervisor group name will result in
constantly eliminating whole supervisor groups and creating new ones
on every deployment, although most of the time the supervisor config
remains nearly unchanged.
This could break existing deployments where multiple Flow instances
share a common supervisor on a server but I don't think those exist
in the wild.