Emails and numbers need to be linked to individual records with a way to specify the type of contact (i.e. main/billing plus things like secretary/bookkeeper/signatory or a free text field) and allow these to be cross linked across different records as needed without requiring it to be linked to an orphaned (no address type ticked) shared address.
When you do want to share a contact IRIS forces you to create a "shared" address which results in records having multiple addresses (many orphaned) where one contact has different contact methods depending on which (group of) companies they are linked to.
The whole system of addressess and contact details is a mess in IRIS. Certain addresses when linked will also transfer across any numbers/emails linked to that address instance, e.g. on import from CH. other times they won't share - possibly when using a person record's address as the shared link.
Many companies require that any communications/documents be sent to multiple contacts - there's no way to record this on the main contact tab - and it is ignored anyway because half the contact details are listed (hidden) within the numbers button on a particular address instance that may or may not be ticked as "main".
List usage often doesn't show all linked records either.