I think the point is that it's easier to use techniques that keep the mail folder always in a consistent or recoverable state, such as journaling or copy-modify

I think the point is that it's easier to use techniques that keep the mail folder always in a consistent or recoverable state, such as journaling or copy-modify-move, than it is to fully handle out-of-memory conditions. Since the former technique is both easier and can handle problems that the latter can't, it's always preferable.