Another benefit is that when you do net-30 usage based billing, you always have to be concerned that the card might be maxed out or invalid by the time the bill gets charged. With 21, you can be sure that the bill will get paid since the Bitcoin is locked up in escrow until the transaction is processed.
If you wanted to set up a traditional CC, you'd need to write the whole billing and tracking system, and so on. Managing all this is cost-prohibitive for smaller solutions.
Oh, and with CCs there are chargebacks, so you can never be sure that the money will actually arrive on your account. This virtually rules out any kinds of services where it costs you real money to run. E.g. if you want to create mechanical-turk API, good luck. Someone will pay with a stolen CC, ramp up $10k in bills, and then do a chargeback.
That said, your point about chargebacks is certainly a good one.
What makes this more attractive, friendlier, and/or plausible than building a service whereby your users—that is, developers—place a CC on file (via Stripe or whatever), and then just charge them by total monthly API requests in one bill?