Using the chain as data storage, rather than currency, costs everybody, because it increases the rate at which people are discouraged from running full nodes. It increases the costs of that dataset that cannot be pruned, and must be carried for eternity: the unspent transaction output set (UTXO), the list of coins available for spending.
Just a quick intermediate question interrupting this interesting discussion:
I've been using the blockchain for a "proof of existance at point x in time" using the hash of a scan of a casascius coin as private key to generate a bitcoin address. I sent 1 satoshi to that address.
Is that to be considered "misuse of the blockchain"?
If that satoshi is spent (which can be done because the private key has been published) the "data" is "prunable", right?