in favor of what alternatives?
Did you read about the Cryptocat security bug? We learned about it because it was open source. Maybe NSA found out about it even earlier, but at least we did, too, and fixed it. Now imagine the same type of bug was in Skype. Then NSA would be the only ones who know about it.
If you are able to decipher such backdoor in the code, or do you expect the code to have a comment saying /* backdoor */ ?