A control in the Windows Azure Portal top-menu where you can click, enter your password (always), and then select which accounts or items in the portal that you want visible and which ones to hide.

That way you can log into the portal and just see the few things relevant to your current context.

The reason for the login to change this feature is that it is a privacy setting and no one else should be able to access it when your have your back accidentally turned.

Two scenarios:
1) Demoing: When doing demos in front of an audience it would be nice to be able to show only specific accounts or even capabilities inside of the accounts you want to show to the audience.
2) When working with multiple clients and they add you as a co-admin. It would be nice to be able to sit with a client and log into the portal without them knowing about your other clients.

Sure you can use separate LiveIDs here but it's more convenient for the user if they can just use one LiveID for the portal.