Quite a lot of webpages need js to initialize themselves, these days, and I want them to load in background when I middleclick on a link. You probably want (at least some) websites to perform XHR when not focused (to update a news stream or something). I find an opt-in behavior on such a common feature a bit to hard, as a lot of website rely on that.
setTimout and friends are throttled (at least in Firefox) to fire at most once every second, so you won't burn your battery having a graphic demo in the background.
Actually, I'd only white-list Gmail and maybe Twitter. I'd prefer it if most site just served HTML to begin with.
I run with JavaScript and Cookies disabled unless white-listed, and just leave the majority of pages that won't load. Techcrunch, Engadget, and most news sites are so much faster without JS.
I might be ok with a timeout - after 30 seconds of no interaction from me, suspend the tab. Would that address your objection?
setTimout and friends are throttled (at least in Firefox) to fire at most once every second, so you won't burn your battery having a graphic demo in the background.