It just wouldnt be enough to make the quest worthwhile.The quest takes time to complete, time that could be used more productively on other activities. The opportunity cost of that quest is, more often than not, just too high to be worth it.If bandits running out from invisible quest area is an issue, it can be solved by just flagging every group that enters it as a valid quest target. That way player can't just kill random bandits on the other side of the map on one hand and on the other he doesn't have to worry about not catching them inside the quest zone.
The current implementation cuts that cost down abit; which is ofcourse one way to improve it. I mean, you could also just have dramatically increased the reward for the quest.
Even so. Even with the new implementation, my personal reaction when I encounter that quest is pretty much "arghh ffs, not that one"