Why is this an issue though?
There's already more than enough incentive to not leave the instance before it's done, namely that it will take less time than the 30-minute lockout you get for leaving early, and don't forget the fact that most people running it only do it once a week for the coin and cluster, which you need to stay until the end for.
Secondly it's a 24-man, one person leaving doesn't have THAT much of an impact, even if it sucks. Hell you could say this is a bigger problem for 4-man dungeons, which outside of roulette give you no incentive to stay until the end if you get your loot early AND if one person leaves you lose 1/4th of your team, rather than 1/24th.