As I just replied to "cle", some disadvantages doing the way that it is because one can't predict what content ID would be produced. Perhaps the hash of the entire contents of the file could point the hash that is current the content ID would solve this issue. To me, IPFS does not seem useful unless this issue is solved. Also, multiple hashes (different algorithms) of the file could point to the content ID/merkle DAG; so if both SHA2 and SHA3 were both used and one of them had a security issues, then just use the one that is OK.