BIP 0032 "Security Implications" states
One weakness that may not be immediately obvious, is that knowledge of a parent extended public key plus any non-hardened private key descending from it is equivalent to knowing the parent extended private key (and thus every private and public key descending from it).
From what I understand, the weakness is due to the reversible nature of the operation on extended public key (addition). Wouldn't it be possible to address this by using a one-way function like hashing instead of addition?