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.