I have several shared mailboxes that live under a main account (the one that they are shared with) but for me it would make sense to have these mailboxes listed alongside my normal accounts, I.E. able to be moved around, placed into account groups, still be visible if the main account is collapsed, etc… Currently, they are just listed as subfolders of the account they are shared with.
Edit: I should clarify, these are Microsoft Exchange Online mailboxes