This, however, is just a little UI over WebRTC, an existing feature that is accessible to web content already. Hello doesn't add any significant area for new security attacks.
It's possible your argument could be against adding WebRTC itself, instead of Hello, but the topic here is Hello.
This is a very valid discussion. Software engineers would naturally have concerns when a browser built to render web pages gets turned into a pseudo operating system. The risk here is that without focus it won't be a good browser nor a good OS.
[0] https://www.mozilla.org/en-US/security/advisories/mfsa2015-0...