It's a big deal, but not nearly as big as it would be if it affected live streams as well. Vods are much less popular. The top videos of all time have around a half million views or less. Top videos this week have around 40000:

http://www.twitch.tv/directory/videos/all http://www.twitch.tv/directory/videos/week

I get the feeling that this does not affect streams currently because if this were turned on for streams, nearly every single stream would be shut off -- I cannot think of a single stream I've watched that does not have ambient music in the background.

It will affect live streams at some point.

Is this speculation or do you have a source? Your comment is stated as a fact but I haven't seen a statement from Twitch about this.

It seems like a logical progression of things, probably just a bit more difficult to implement. I'd guess it's not long til we see it.

Yup. From what I've heard, Youtube already has something like that on their livestreams.