Personally, it's a neat idea but I don't see it as being very practical.
Wouldn't this slow the site down a decent amount? I don't see enough of a pay-off for that myself. Now, if I were wrong about it slowing things down, then I don't see it being a huge problem (unless it's really hard for the devs to implement). I still don't see a need for it though, but it would be okay if you could choose to turn it on/off.