I think I'm missing something. Wouldn't the alternative be to keep playing the video ad even when you're not looking at it? Why is that better? I don't want my browser wasting cycles to render a video it knows I can't see.
At the risk of stating the obvious, it sounds like you have a problem with ads, not a problem with the visibility API.
Depending on your browser, that seems pretty doable with a plugin. I assumed ad block plugins would nuke video ads anyway -- is the issue that they are preroll and hard to filter?
At the risk of getting into an ethics debate, that's sort of the deal being offered by the publisher: you get the content in exchange for watching this ad.
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.
I like to open videos of talks an play them while I do other browsing and work. I still am listening just not watching. If the video stops when I move away, now I can't have my background sound be interesting info about $TOPIC.
At the risk of stating the obvious, it sounds like you have a problem with ads, not a problem with the visibility API.