I know you will argue that then some groups will fall apart after wiping and no one new will join, but you've already addressed this problem with the stacking buff you get after wipes. And lets say I actually get to do the first boss this week, as soon as that boss goes down I'm going to leave, and someone else is going to get stuck joining a group in progress and continue this cycle of annoyance.
Perhaps the best solution would be to make it so that if someone has downed a boss already - and gets put into a group that is currently on that boss - then they can see how many bosses are down and decide if they want to join or not. Then on the flip side of that, if they haven't done the boss, they don't get to see the current progress of the raid before joining.
Anyways, rant over, hope someone at Blizz actually sees this.