It prevents a whole lot of pipeline and concurrency optimizations since it forces you to rehash every file in the change set, before your rename detection algorithm can kick in.
For example, if a user renames a root folder with tens of thousands of descendants, then the hash approach would have poor time-to-first-change-synced latency.
Furthermore, relying on file hashes won't actually work.
A user could have done both a RENAME and UPDATE on the file, since the sync app last ran (you need scanning logic for this, and can't rely solely on inotify). To survive a RENAME and UPDATE, you would then need to compare partial file hashes. But again, it affects latency too much.
