Starting in late July, Microsoft will be eliminating the "linked accounts" feature from Outlook.com. While this feature has been used for many reasons, particularly where you don't want to provide your primary email address, security is the primary reason Microsoft is giving for this change.
According to the announcement by Eric Doerr in the Outlook Blog, it has been increasingly found that linked accounts are less secure than using aliases, particularly because it is possible to sign in to Outlook.com on the web and then switch to any other linked account without entering a password.
More in my blog post,
Linked Accounts Being Eliminated from Outlook
