One interesting scenario, mentioned I think elsehwere in the comments to this article: a single 'misplaced write' (i.e. disk doesn't do the seek to new position, writing to old position) means that a data block goes into the ext3 journal.
In the absence of ext3 journal checksumming, and if there is a crash requiring replay of this journal block, horrible things will happen - presumably garbage is written to various places on disk from the 'journal' entry. One symptom may be log entries saying 'write beyond end of partition', which I've seen a few times with ext3 corruption and I think is a clear indicator of corrupt filesystem metadata.
This is one reason why JBD2 added journal checksumming for use with ext4 - I hope this also gets used by ext3. In my view, it would be a lot better to make that change to ext3 than to make data=writeback the default, which will speed up some workloads and most likely corrupt some additional data (though I guess not metadata).