First:
You will not get loot on boss that you already killed, only on the boss that you didn't kill.
Second:
People fail on a pug and don't log anymore, at some moment you got less and less pugs because everyone is saved, wich is bad for the gameplay.
3:
Blizzard is using this system already.
4:
cons: people gonna leave more often
pros: theres more people to join as they don't get locked, they log more in game to finish it, don't need to extend lock for the next week, more fun less frustration.
AND BRING BACK KARAZAN EVERYONE LOVES IT AND WHANT TO PLAY IT !!