I have not found much benefit to assigning the top level bug or product backlog item to an individual… since each one has a set of tasks associated with it which are assigned to individuals. Once each task is done I mark the top level item as done.
I’m wondering what the benefit of assigning them to users is?
I can’t think of any benefit to the developers having both, however.
Project Server Integration
Some people only sync the PBI/UserStory level to Project Server, so assigning these to people can be valuable to project managers using project.
Owner not Doer
Something else I’ve seen is people assigning the PBI to the person in charge of keeping it up to date, or just whoever had the original idea. This way people know who to talk to if there’s any questions about the PBI.
Personally I leave them unassigned too.