No, mail spools/queues do not need replication. Stuff in the queue are usually deleted in a second and such dynamic change is not worth replicating. If you do put the queue on a distributed filesystem, in most cases you cannot have more than one instance running save for sendmail.
outbound mail can sit in queues retrying for hours negotiating their way into the greylists of the likes of Yahoo. I guess if you don't mind the possibility of messages getting lost around a server failure event, then its no big deal, for sure.
That only leaves putting the queue on a distributed filesystem. Replication will not be a complete solution to that and qmail queues cannot be replicated.
Then there is the question of whether to leave a message in the queue for more than 4 hours given the expectation of email to be more or less instantaneous.