Almost every bzr operation can operate on individual files, e.g. add, rm, log, diff, commit, revert, etc. The shelve command is no different, as it can take a list of files to shelve. However, the unshelve command is very different, and it's currently possible to unshelve a single or group of file's changes. It seems like this should be possible.
One big driving scenario where this would be useful is when you have a large tree full of changes, and you run a "shelve --all" to put them all on the shelf. Then you want to pull out the changes on individual files or groups of files and work on them individually. Currently this is impossible, unless you unshelve everything and re-shelve everything you DON'T want. When working with hundreds of files, this is very impractical.
I think this would be good, but it would be a bit complex to do. Mostly because you need to stage a new shelve-item which is the difference between what you actually unshelved, and what was left behind.