It would be very useful for integration with custom build and release cycles if the REST API was able to create and delete branches and tags.
One of our larger products runs to 300 repositories. We use a script to branch and tag them all with the same names. At the moment we have all our git authoritative repositories on a network file system so we can simply cd in and tag.
As we move to Stash however this becomes problematic. Without API access to this feature it requires the user to have a local clone of all of those repositories. If they don't you'd have to clone, branch/tag, push, delete. A slow process over this many repositories.
We can't guarantee a developer has the whole system cloned as our build system is designed to let them only clone the parts they are actively working on.