I wonder, if when someone they know to be a "good" person decides to leave -- not because they've been offended/stumbled, or had an affair, or started smoking, or wanted to become a wealthy ceo -- what does that do to a still-in jdub's thinking process?
While most will call Satan's world as the culprit, or hidden sins (cause why else would you leave the borg), surely there is at least one person with some residual thinking ability who gets jarred into doing some research.
Because when a good, intelligent person leaves and they stay... surely there's not an easy way to restore balance to the neurons going crazy in their brain. They may not take instant action, but that nagging question might just be added to many more like it to make their exit a future reality.
edited to add as I just saw this: As Vidiot said more succinctly: Because they hope, deep down, to prove to you (and therefore, themselves) that it's not.