I dare you to quote me on this years from today.
NEVER EVER GOING TO HAPPEN!
Here's why: An automated system intended to punish leavers would be prone to abuse and a non-automated system would require Blizzard to dedicate resources, as in hire more customer support staff, to police players. This is Blizzard we're talking about, the same company who some time ago fired most of their customer support staff because they're greedy sleazebags. Hell will sooner freeze over before they hire more people just so that people won't have to deal with leavers as much. Their priorities are profit and profit alone, your enjoyment of the game is a secondary objective at best. As long as you don't quit in large numbers it's all good.