The purpose of security is to prevent unauthorized people from accessing the account. There are tons of accounts that are legitimately shared, and there is nothing wrong with sharing passwords in those situations, if the account doesn't have any technical mechanism to allow for multiple users/profiles on a single account. For example bank accounts, utilities, Netflix, Hulu, wireless router administration, all have been shared accounts with my wife (some have since added profiles, but not all).
Furthermore, even with accounts that we keep separate, like email, there are useful reasons to share the password, like when my wife is away from internet at work and wants me to print a boarding pass that was emailed to her. Sure I could snoop through her email, but I don't just like I could snoop through her purse or journal, but I don't.