With the release of 8.0, one of the buzz-words is "heavily focusing on usability". That said, please don't scrap the Bookmark feature as I've gotten hints that you may be considering!

If people are not using it, it could be because it's not persitent between sessions - which is a fundamental flaw of course, but one that should be easy to fix...

Yep, they are only marginally useful if they are gone the next time you open the client.
But they would become REALLY useful if you can continue the next day where you left off.
NOTE this is not about labels - bookmarks are great for temprarily (and locally!) tagging certain changesets in individual workspaces, while labels are global.

Currently, all applied bookmarks are gone when restarting the Plastic GUI.