How secure is automatic authentication in Skype?

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http://superuser.com – How secure is checking the automatic authentication option in Skype client? What if I sign in with Microsoft account instead of Skype id? What if both accounts are merged? Is there any documentation on how it works in recent versions? For example, whether it stores a plain text password, or how easily it can be retrieved. My concern is that the Microsoft password is just too important, giving access to all Microsoft services, while Skype account is more specific. I do not want my main password having any chance to get compromised just because I do not want to keep typing a password all the ti (HowTos)