Would you do it if it was cheaper per kwh? Say you have an EV where any battery you get from the swap station is better than 90% degradation, and you pay $0.40 per kwh for the electricity in the battery- but you get the option to take a battery between 80% and 90% degradation, in which case you get the battery all filled up, but only pay $0.35 per kwh for the electricity in that battery.