As it stands, any postal or email addresses contained in a constituent record which are not tagged as "default" are hidden from display and export. Which means that there can be usable contact data that appears not to exist from the user perspective when creating mailing lists or performing data hygiene.
This is not a good thing. Any existing postal or email address should become the "default" address by default. If the user wants a constituent record to hide an address for some reason and to have no default address, that should have to be done via override.
Also, if there are any other postal addresses, emails, or phone numbers stacked behind the default, this should be visible on the user interface so that the users knows they exist, and records containing hidden data behind the default values can be searched for.