It is difficult for Juju charm authors to support forked charms. dev-ops often don't realise they are forking a charm when they add files that they want the charm to deploy. We cannot assume that the deployed charm's revision (or the version control revision number) will ever match what the author released. I use Bazaar tags to mark the versions I support and help identify the true version of a deployed charm.
Juju uses the charm's "revision" file to store the version. The number in this file must be incremented to deploy additions or changes to the charm. Upgrading a deployed charm requires a higher revision number. Organisations commonly create a local charm repo (managed by a version control system like Bazaar) to freeze the versions they deploy. These known version are stable; this is a best practice when working with charms that change often. The local branch will get commits that are not in the branch published at Launchpad. The revision numbers are meaningless between the two branches.
I tag my charm branches with the charm revision number. When a dev-op forks my charm to add it to the local charm repo, we can compare branch tags. For example, I tagged lp:~charming-devs/charms/precise/elasticsearch/trunk r39 as elasticsearch-29 to match the number in the revision file. I can ask the dev-op which tags the local branch has. I can quickly identify missing releases. I can ask for a diff from the tag I last released to help diagnose problems with the changes.
For example, several issues were reported about my elasticsearch charm. I had addressed the problems months before. I suggested the dev-ops upgrade to the latest version. Since their version numbers were higher than my versions, they did not upgrade. Tags are more precise. I can suggest the dev-ops merge "-r tag:elasticsearch-29".
Another case, charms may need to change often if the application is deploys is under active development. A recent deploy of charmworld failed; the charm upgrade errored. The dev-op and I couldn't reconcile the version numbers between what was deployed and what I released. We had to read a diff of the charm's directories and files to discover the deployed charm was missing several releases--the charm was incompatible with the deployed code. If I had tagged my release, we could have identified the issue in minutes.