i dabbled in this api way back in the past so i may be wrong about its capabilities.
skype used to be EXCELLENT at working in most networks, including "locked down" corporate ones. Network admins used to find it notoriously difficult to "ban" on networks.
so relying on skype to exfiltrate info may serve two purposes:
1) use another program's capabilities instead of reinventing the wheel.
2) hide the fact that some random program is doing network access.
skype could be one of a range of data exfiltration mechanisms with different levels of obfuscation.
Well, if you have any of the closed-source companies' software on your system (and by definition, that is +/- 310mio citizens, in the US alone), you are sure to have NSA backdoors on your system. Such backdoors certainly do not require manual intervention for them to be exploited on large scale.
But more to the point, you don't know what's going on in closed source code. It's trust. However the trust can, and has been violated in the past (whether by the provider or by a third party is immaterial). You just don't know. Now that doesn't mean that open source software is immune. I seem to remember there was a backdoor found in the Linux kernel a few years ago. These things happen, but at least it's easier to audit.