I think there should be a penalty for withdrawing. Nothing too harsh, like 2-5 minutes with no DF use for withdrawing 3 times in a row, and each subsequent withdrawal accumulates more total time per week or something. So first offense is say 5 minutes. Withdraws 3 more times = second offense: 10 minutes. Third 15, etc. It would deter "in progress fishing" and it's not much of a penalty. If they get in and leave duty it gives them the 30 minute wait, which is a far steeper penalty. Just a quick thought, kinda just popped in my head seeing this thread.

The join in progress button is there so if a duty gets partially abandoned for whatever reason the people who started the duty aren't completely screwed out of completing it, not so "I can get a quick run to get X gear or tomes with little to no effort."