Two reason why I have ended up using RDBMS for messaging: 1)Transactions can easily cover both DB and messaging operations (distributed transactions are not easy/possible on all platforms) 2) The database is anyways there, no need to add another critical component to the stack.
I think a better reason is that jobs and messages can be relational with the other tables in the system, and so can provide the same guarantees that the rest of the db offers.
