Another reason having all the source code is a good thing. It stops people being able to add spyware in places you really don't want it.

http://arstechnica.com/security/2013/05/think-your-skype-messages-get-end-to-end-encryption-think-again/

http://www.h-online.com/security/news/item/Skype-with-care-Microsoft-is-reading-everything-you-write-1862870.html