If someone has stored all their precious photos and media files on a disk, and the metadata is trashed, then rebuilding the tree should get them their files back where a regular fsck wouldn't. I wouldn't trust --rebuild-tree not to add random files at the best of times, for example, I understand that it restores deleted files  which you probably don't want to do in a routine fsck. If, on the other hand, you've just found out that all your backups are on write-only media, rebuilding a tree from leaves could save you from losing years of work. It would be even better if it didn't merge partitions, but is still better than nothing if used as a last resort.
I think it would also be better if it encouraged you to rebuild the tree onto an entirely new partition.
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