It's better because the user doesn't have to sit on that tab waiting for the ad to play out. How it currently is one can go check Facebook or HN (or even do some work!) while waiting for the ad to finish.
To move beyond the ad issue. Maybe I'm not interested in watching the video (say a presentation) but only listen to it. Then I have a legitimate reason for not wanting the video to pause: doing other work in other tabs.
Which is why it's good for marketers but not the user. I don't need to see the same ad more than once in a session but many video sites will display the same handful of ads every couple minutes. This is especially prevalent on twitch.tv when watching gamers stream, it's usually the same ad every time. After the first time I see it I'll just visit another tab while it's playing (mind you the audio is still playing.) If they were to pause while I was doing this I wouldn't bother watching at all.
