It isn't about trust, it is about providing predictable behaviour in all circumstances, even weird corner cases... So maybe that is about trust - you should be able to trust the unionfs to behave predictably.
In your taxonomy of sys-admins you forgot to include the brilliant/insane ones who *know* exactly how every union-mount is being used and *knows* that a particular file that they want to upgrade isn't being used at the moment so if they replace it on the common underlay then everyone will smoothly see the new content.
To serve their interests you want unionfs to perform predictably in that situation, so that if they try something and it works, then it is likely that it will work again next time. So it is important for unionfs to understand and handle any changes in the underlying fs.