since the list of subscribed to labels in the repo record is simply a list of at-uris theres no reason we shouldnt be able to subscribe to other peoples custom labels. this is even effectively how the default labels work (being custom labels that are owned by the tangled.org account)
this would be particularly useful for bots that add labels but i imagine also very useful for community maintained sets of slightly less limited/opinionated labels than would be appropriate for the list of default labels
Another common usecase: reusing standard labels in your own repos.