What they need is two check boxes.

- Allow In Progress
- Join In Progress Only

If you don't select either, you get a fresh run only. If you select 'Allow In Progress' you can get either. If you select 'Join In Progress Only' you will get In Progress only.

Why? Because there are people who absolutely do not want to do a fresh run (for whatever reason) and so giving them the Join In Progress Only option means they will stop trolling people who do want fresh runs (the people that keep withdrawing will not simply stop, so quit thinking that). However, there are some people (myself included) who will queue for Allow In Progress because I don't care which I get and because it means that if someone leaves a group already going, they still have the largest pool possible to fill that spot so they can continue.

Awarding end-of-instance bonuses (i.e. tomes) based on what percentage of the dungeon you complete doesn't solve the problem; it just means that if someone leaves your group you will most likely be SoL because then people won't queue at all for In-Progress runs.