The problem is if one person gets to AFK and get rewarded for other people bothering to do the work, why should anyone else be expected to do the work? There's no incentive other than finishing it faster, and if you can just AFK the speed it takes won't matter because you're off doing something you think is more fun.
I think they should impose the same penalty to those who afk out of BG's, LFG's, LFR's, and Warfronts as those who leave a LFR/LFG, as in an 1 hour penalty before they can join another. That way they do start taking things a bit more serious like those who are attempting to complete them.