The upside is maintaining a good and respectful relationship with your past employer and/or past boss. Just because you're leaving does not mean you don't want the company to succeed without you. If the person you're giving feedback to (assuming the feedback is given thoughtfully and respectfully) is going to be thankful and receptive to it and it has the potential to do the person and the company some good, there is value to you.
Another upside is that your feedback might make things better for your ex co-workers, some of whom may be your friends. And if the company is afraid enough of losing more people, they might actually take your feedback seriously.
Agreed, but it is a high risk low reward thing so it is bad as a general advice.