When a user has access to 2 projects (ProjectA and ProjectB), he will be able to add new labels in projectA and after that, when he make the same operation in ProjectB, it will show that the label is there (clicking on the arrow search button), he just need to add without create a new one like he did in projectA, but doing that operation could be misunderstood by projects admins where they prefer maintain a label related to a project, so any users who just have access to projectB are able to see those labels added by any user who has access to both projects.

So it would be a nice feature restrict labels to projects instead of users in order to have a better permission organisation.