It's certainly the case that if they're leaving because they're unhappy that something is true and it is still going to be true when they consider returning it would give me pause. :-)

Even then, it might be possible to convince me if they come back with a reason they were wrong and think the thing is good (e.g. I tried Agile and discovered it isn't better, and my actual problem was this...) rather than just, "I decided to put up with it here..."