The data remain 100% auditable, even if it is stored by third parties.

With the block headers and Merkle proofs leading to the transactions, this gives cryptographic proof that the data is included in the blockchain. So someone else can store the data, and provide it when needed, without the other party having to trust that it has not been tampered with.

If someone wants to audit all the data in the blockchain, that is their prerogative. Others may want to audit only portions that they are interested in.