Perhaps because your example fails to describe what happens if there is a crash, after updating the database to say "email sent", but before the email has actually been sent.
Well, I didn't want to go into that much detail here, because that also presents a problem in the case of locks (if the locking process crashes, the lock is never released).
I like the fencing solution. Couldn't such a scheme be implemented over a database instead?
Why I'm thinking more in terms of a database-oriented solution is because there is still the problem of crashing. What happens if the system crashes just before the email is sent? The system somehow needs to remember to restart that task (sending the email) when it comes back up. And this is probably best done through a database anyway.
