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Only data that is strictly necessary for the world to validate a transaction has a place in the block chain. That's the whole point of it. Everything that is only significant to the sender and receiver (or miner) should be between the sender and receiver (or miner) and doesn't need to be stored forever by every other full node in the world.
I think there are definitely use-cases for associating some immutable meta-data with a transaction. Example: a bitcoin client that gave a unique refund address for every outgoing transaction, and automatically groups refund transactions together with the original payment transactions.
Somebody could create a service that associates data with transaction ids, but they need to do more work to make the data immutable... and it is not clear to me how you make that secure.
I really want my refund address to be 'baked in' to the transaction that I sign, so if the transaction is accepted into the block chain I know there hasn't been some hacker somewhere who managed to rewrite the refund address so they get my coins.
If I'm doing some type of smart contract with bitcoin transactions, I want the contract data baked in and covered by the transaction signature. And the person I'm transacting with would like to be sure I can't change the terms of the contract once the transaction is signed.
It seems to me the simplest, most straightforward, and secure way to do that is with a limited-data OP_DROP transaction type. The data in the blockchain is (transaction+HASH(metadata)), and that is what is signed. The actual metadata can be stored outside the blockchain and looked up (and verified) by hash (hand-wave, hand-wave, I have no idea how that happens, if there is more than one place that stores transaction metadata, etc).
Any scheme that tries to move the HASH(metadata) outside the transaction signature recorded in the blockchain will, at the very least, be more complicated. And, therefore, very likely to be less secure.
Am I missing some other simple, secure, decentralized, non-blockchain scheme for attaching metadata to transactions?