I don't usually queue in progress though, I guess that makes a difference, but even when I do 9 times out of 10 I just arrive in complete parties. I think you must just be very unlucky, I very rarely see an instaquitting tank or healer (or ragequitting, or even rage in general). The occasional d/c, sure, but even those are usually part-way into the dungeon.
Strange how experiences can be so opposite.
