The purpose of the bitcoin block chain is to provide an imitable public ledger.
If you read the original bitcoin paper you will see that the block chain is but one component of a larger whole that provides public trust that coins were not double spent. See section 2 "Transactions" that states this explicitly.
Within a traditional bank the ledger is private and transactions are not signed by the owner. The trust in this case is somewhat blind but revolves around a promise by the bank that it will handle transactions on your behalf with care, and after all it is in the banks own best interests to prevent double spending.
Why don't banks use a bitcoin style ledger, well a few spring to mind but I will admit to a little speculation here.
- Block-chains are computationally expensive to check continuously and their are much faster book keeping methods borrowed from traditional accounting practices.
- Database transaction logs and transaction hashing is available to prevent corruption of data in storage.
- The ledger is private so the only entity the bank needs to satisfy the correctness of transactions is itself.
- If the bank were to use a Block-chain then where are the private keys to sign transactions going to come from? Is the bank going to store them for its customers?
Lets now look at the article you cite from Iron Mountain. This is a slightly different kettle of fish. They store data that you CAN sign and while we expect that data to be private between the owner and storage provider the trust equation here is different. The bank was self incentivized not to duplicate or modify transactions because it would directly loose money, in the IM storage case - aside from paying them monthly fees and perhaps reputation - we have no implicit reason to trust that they will not modify information. So for IM storage we do have a case for the use of a block-chain.
The question of 'are digital signatures enough' depends on
- You use a decent key size sufficient to provide perfect forward security.
- You trust all members of your organisation that have access to the private keys.
For archiving purposes a block-chain can also prove that one document preceded the next, something that digital signatures alone can not do. It can also show that no record has been tampered with by users with access to the private keys.