While its true that not all turnover is a bad thing, and some people really shouldn't continue with the company, it lowers our corporate discourse to speak ill of the (vocationally) departed. Its an easy out; blame someone who had more than their fair share of human or career flaws and who is no longer here to take up for themselves. It's also unfair and can come to no good end. If the criticism is too harsh you may cross the legal threshold into disparagement. This is actionable on the part of the former employee. If the criticism is personal and distributed in writing (remember, email lives forever!) it can cross the threshold into slander.
If it isn't that bad, its just cheap and should be beneath us.
Remember, this industry and our company have a long history of recycling people, and today's whipping post may be next year's author, or customer, or boss. It has happened before...
As with most business situations, the morally correct course is also the smart business move; don't say anything about someone after they're gone that you wouldn't want said about you if you were the one to leave. Don't put anything in writing that you can't defend with facts. The Japanese for whom I once worked used to say, "Fix the problem, not the blame". Similarly, if something is wrong lets fix it and resist the temptation to blame it on anybody, much less those who aren't here to respond.