Have you ever been in a mediocre pug, suffered through repeated wipes, AFKs and similar annoyances just to stand before the last boss and think, "phew, almost done"? And then you try to kill that boss, and what with the pug being mediocre, you wipe due to the dps being lackluster, the healer being undergeared or some similarly mundane reason - either way, not exactly something that's impossible to overcome. But suddenly... the tank leaves, the healer leaves, and before you know it, your chat window presents you with the words "your group has been disbanded" and the dungeon finder happily offers you the option to queue for a new instance.
You know what? Screw that. Seriously. After suffering through all those nuisances, I have a right to complete this damn instance. It's not my fault that the system grouped me with a bunch of spineless imbeciles who run at the first sign of trouble. Why punish me for their failures by forcing me to start over completely, even though I'm right in front of the last boss and ready to kill? Not to mention that I bet the dungeon finder is full of people who'd be more than happy to score a run that's already at the last boss to get their daily points dose quickly. Seriously, you should be allowed to replenish your run in progress from the dungeon finder even if you're the only person left in the group.
The problem isn't limited to max-level dungeons and point farming either. Nothing like queuing for a specific instance on a low-level character and getting in after half an hour, just to see the tank charge off before the healer's even zoned in, people drop group to avoid the wipe and before you know it the group's been disbanded again before you've even killed anything. Really, offering me to wait another half hour for the next run (which might end up being just as bad) isn't even a cold comfort. I already paid my dues in terms of waiting time, I should get priority to fill this one-man group and complete the run.
Also, since we're already talking about annoyances related to people leaving prematurely, what's up with the loot rolling at the end of an instance? Don't you just hate it when someone is really slow to roll on the last couple of drops, half the party leaves and when the roll finally goes through, it says that the item was won by a person who's already left both group and instance and will thus never get it? Once again Blizzard laughs in the face of people with patience, as they have to watch their desired drop get assigned to someone who hasn't even bothered to hang around long enough to pick it up. I think that whenever you leave an instance group, you should automatically forfeit your right to any loot rolls in progress so that the item can only go to those who are actually still around to pick it up.