[crandles]

As a feature, I think it has a lot of merit. Advertisers will love it, as it actually improves on TV ads (which you can fast forward through easily).

If you're already using some kind of ad-block, there's nothing stopping you/your ad block from disabling the visibility API on select pages (youtube et. al). It's a Javascript API, and that means you'll be able to alter its functionality fairly easily (bookmarklet, plugin, addon, etc).