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If you are running with journaling enabled, you should get a much stronger crash recovery case.

In the event of a crash, the journal will be replayed: http://www.mongodb.org/display/DOCS/Journaling

And that is a good feature on a database to have! Why not make it the default?

When we added it, being a new feature the decision was made not to initially flip it to default in case of bugs, unexpected behavior, etc.

As noted, it's possible that will change in future releases; there's been a lot of improvements to journaling since its initial release.