I totally get your point of view, particularly if you zone in for a single boss that, even if successful, means you will soon be back to waiting... again. However, as DPS, entering in-progress means you may have a competant group that removed a sub-par player (or jerk), and nothing more.

But what about other roles? On my baby warrior (~lvl 41), I zoned into SM Cath last night at the doors to the cathedral itself. My first thought: "Oh shit, previous tank bailed." And my thoughts were pretty well confirmed by a couple DPSers who decided they wanted to pull packs on their own. But guess what?

I was determined to right the ship and finish the job. And we did, death-free. Granted, my example is one of revamped Vanilla dungeons, not a Cataclysm heroic, but the principles are the same.

Let's not even get into zoning in-progress as a healer... /shudder.

It would be nice if some sort of concession were made to people of any role (though tanks need it least of all with insta-queues) who finished in-progress dungeons, like moving them ahead (not to the front necessarily) in their next queue. It would be an incentive to suck it up and forge ahead, but I can't imagine how to implement such an idea.