When is a .gemlhistory sidecar actually worth having?

[xiongjy2104]

geml history save records a revision beside a document; geml revert rolls one
block back. Nothing in the docs says when that is worth doing — so the default
a reader infers is "always", and every file grows a sidecar.

The practice that emerged here is narrower: a document git already versions
usually does not need one. The sidecar earns its keep where you want rollback
between commits, at block granularity — a spec being revised over many steps,
or a document an agent edits unattended while you are not watching. Config files
checked into a repo do not qualify; this project excludes them explicitly, and
the repo is quieter for it.

Would you want that written down (a README line, a non-normative spec note),
or is it obvious enough to leave to judgement? And would you want
history save to say something when you run it on a file git is already
tracking?