No everyone would get it if the dungeon is unfinished. The thing is that if you are one of the people who joins a key for 5-15 mins and leaves you would get many many more over time than someone who always tries to finish no matter what. Then you could look at applicants with:
Timed +15 runs 50
Unfinished 15 runs 250
Timed +15 runs 50
Not timed +15 runs 225
Unfinished 15 runs 25
Who would you pick? I suppose someone will think that the guy with only timed 15's is better.