This explains me 100%.
Every time I am online I do this for AK, WP, Prae, and Brayflox as a 50WHM. I don't join Prae, WP, or AK unless they are at the last boss. I don't join Bray unless they are beyond the first boss. Why do I do this? Due to class and work I can only play starting late friday through mid sunday (some weekends, like this coming one, I can't play even that much due to presentations and meetings). I want to finish what I can as quickly as I can so I can catch up to all those players who play everyday all day. Unless they implement severe punishment for it I don't see myself stopping this for any reason anytime soon.

You honestly shouldn't be complaining about me backing out of a Duty Finder party before it starts.
Let's say I, as the healer, back out after we are all matched. If this happens, then the next immediate healer that registered after me will take this spot. If the other healer that took this spot decides to enter, great! If they, too, back out, then it will be passed to the next immediate healer. This continues until some healer joins and all is good.
But what if there were no other healers searching but me? If that happens, then the remaining three players who were searching are put at the top of the queue and enter as soon as a healer with more free-time appears. This situation is exactly the same as if I was never searching to begin with. I assume I do not have to explain the bold to you.

In my opinion the developers should not be upset about this, but instead add a drop down menu to the Duty Finder next to "Join in Progress" that allows players to only match with parties that have cleared [0/4+], [1/4+] [2/4+] [3/4+] of the dungeon. This way we don't get matched with new parties and you don't complain about us and instead complain about the already existent wait times for the Duty Finder that you suffer anyway.
Even better: I want the developer's to also add a function that tells those you join in progress the current number of player roles left in the dungeon at the time of matching. I'm tired of joining AK/Brayflox at the end without a tank. It should mention "Progress [1/4] Tank[1/1] DPS[2/2] Healer[0/1] when I try to join in progress.
In fact, I like this idea so much I will post it as a whole new unrelated thread so it can get some attention.