We have many couples who donate and/or volunteer who share a single email, which creates problems when the system does not allow modifications to one or the other record because they have a common email. The only solution is to remove the email address from one of the records, but if we need to get in touch with that person individually for some reason there's then no email on record. Since the system uses unique customer IDs, there's no reason not to allow constituents to share emails and to allow updating and modification of constituent records containing a shared email.
We are running into the same issue. An easy fix would be to have login credentials with something other than emails.