People using this system to get into runs where a tank/healer DCed/had to leave just before the final boss are just being jerks. Nothing more, nothing less and they really shouldn't be trying to justify their behavior.
Pretty much this, I always wondered why (in particular tanks) did this to groups in DF. You get maybe 10-15 withdrawals before getting a full party. It's got better recently but was really horrible in the first month or so of release.

Now that I know the reason, I'm dumbfounded that people are so lazy and try to justify wasting other people's time to serve their own purposes. That's really what it boils down to. Not to mention that fact that the time people waste looking for an in progress group could be used to actually complete the dungeon normally. Don't know what the average time for finding an in progress group is, but it must take quite a while sometimes.