adCenter does not have a true My Client Center equivalent, nor does Yahoo. I have told the folks at Microsoft two years ago that they needed this feature if they wanted to encourage more agencies to start adCenter accounts for clients.
The problem with their one-login-multiple-account system is that it is not clear how to link and unlink accounts, and I am also concerned that financial liability may "slosh" from one account to another. Until those issues are clarified, I advise people to set up their own account and then create a login for their account manager. That way they can keep ownership of their account, and cut off the manager when they no longer want him to have access.