Dungeons - meant to be played until the end.
Loot - meant to be a reward for the effort put into a dungeon.
Quitters - not playing the content as intended, i.e not playing the dungeon until the end, i.e abusing a broken system.
Remaining group members - Punished for playing the game as intended, by waiting for a new member and for receiving loot slower than people who loot and run.
We know Blizzard wants us to play until the end and not leave our groups, because they reward the former and punish the latter. Only, they obviously aren't doing it enough seeing as the problem persists.
It is that simple.
Now, if Blizzard were to fix this, for instance by increasing the reward for remaining in the dungeon until the end, thus making the time invested in it worthwhile and more beneficial to a character's progression than quitting halfway through, what on Earth is your issue? Why are you defending a broken system simply because it works for you?
If, let's make a very rough estimate, about half the playerbase is slightly annoyed by quitters (evidently, most people do not quit, we can all testify to that because it's always the one guy who leaves), then I'd say it's a very appropriate thing to do to solve the problem by slightly altering the design. You quitters will be happier, the rest of us will be happy, and the issue will go away. M'kay?