Windows 10 needs to have office 365 accounts as "Microsoft Accounts" & use of personal domains outside of business accounts. A normal user does not want to use Lync or SharePoint, They want there domain and have the normal commercial applications.
At the moment it is hard to have "ONE" experience with being in the Microsoft Ecosystem. You have your Microsoft Account to sign into windows and windows phones. Then if you want to have a custom domain or an office 365 subscription you have to create another account that you cannot use to login to windows or any other system (Xbox, Windows phone... etc)
These accounts need to be fully functional windows accounts to make it one account for all systems and devices.
Moved from Insider