Rebuild tree a useful feature with side-effects.

The fsck for ext2/3/4 doesn't have this feature because it doesn't need it. One of the tradeoffs of using a dynamic inode table (since in reiserfs it is stored as part of the btree) is if you lose the root node of the file system, you have no choice but to search the entire disk looking for nodes that appear to belong to the file system b-tree.

With ext 2/3/4, we have a static inode table. This does have some disadvantages, but the advantage is that it's much more robust against file system damage, since the location of the metadata is much more predictable.