It's common to take a departing employee out to lunch in my group, as recognition for the work they'd done while they were here. In the late 90's when the boom was in full swing, our group had to stop this practice because of the high turnover rates. Having a goodbye lunch every other week was not exactly great for morale.
It does establish a strange sort of dynamic. People who leave appear to be rewarded for leaving. For people in a stressful period at work or in a very active job market, they might see that as a prize to be won.
I wonder if there is there a better way to show the departing employee that you appreciate their hard work without making it seem you get a reward for quitting. Maybe a "private" lunch between the employee and their supervisor so that it's not such a group event, while still allowing the opportunity to honor the employee's hard work?
I don't think it's really a big deal. I don't see people leaving because they will have such an exciting parting event. It just seems to me like it doesn't necessarily send the right message.