Could Amazon be theoretically more efficient - sure. In fact, I wouldn't be surprised, if over time, the time requirement for owning (30 days) and the round of

Could Amazon be theoretically more efficient - sure. In fact, I wouldn't be surprised, if over time, the time requirement for owning (30 days) and the round off period (To the month, rounded down) and the fee (12%) is rounded down - but, this is a new service that did not exist before - so from that perspective, it's great. All the things that you used to be able to do (lend someone your EC2 instance) still exists.