I think it makes a lot of sense to have a spare camera body. Especially if you are going on important or once in a life time trips. Less so if you dive mainly at home.
For the record I've never owned a spare body for any of the Nikon SLRs I have used. And never had a flood or been let down by one (and done 1000s of dives, shot a handful of books and hundred of magazine features).
But I have seen many other people flood theirs. Especially newer underwater photographers are most vulnerable to making the mistakes in housing preparation that lead to most floods. If you look at the problem logically, it is impossible to argue against the sense in having a spare.