There are a couple key problems:
#1:
Originally, when abandoning a Duty, you only took a 15 minute lockout. That's not so bad, but prevents you from dropping instantly every time. Now it's an incredibly long 30 minutes. You'd think this would prevent people from dropping, but the reality is that now people will just troll you/go afk/etc. until you dismiss them. The problem here is that you can't add a timer to people who got dismissed, because then when it's used unfairly people are going to be furious, and also now instead of the intent of making people stick with the run instead of eating the timer, people are now actively hindering the run and actually making it worse for the people who want to do the run instead of better.
This happens... a LOT.
#2:
You can't "Vote to Dismiss" while fighting, or when already voting on someone else. This can lead to "fun stuff". Best example:
In Syrcus Tower, there was a healer who intentionally locked himself out of the Xande fight. Since when the lockout occurred we were clearly fighting, we couldn't vote dismiss them. Couldn't vote dismiss them after the fight either, because the dungeon had been cleared. So, as long as we win, he gets a free shot at loot without doing anything at all. Does this seem somewhat unfair to you?
The game shouldn't actively encourage poor behavior, and yet it does.
#3:
I can't believe I almost forgot this one!
There's absolutely nothing to prevent someone from getting dismissed, then queuing back up for that same dungeon/instance, and rejoining your party right after they left (assuming they selected "join in progress" and here's not someone else of that same role who also checked "join in progress" who queue'd before them). I was once in a Stone Vigil (Hard) where the healer refused to heal, and also tried to mpk us by pulling way too many monsters. He was kicked by every single member of the party, and rejoined the dungeon in progress THREE TIMES. We just gave up and abandoned duty.
This will happen a lot with tanks and healers ready to troll your dungeon/primal runs.