It's also bad, as it means that if, say, someone gives an inspector the wrong card (and doesn't realise it at the time), this can't be sorted out until much later.
How long is "much later"? It can be sorted out the following day, probably on the phone to the helpdesk, as I'm sure that a second contactless card owned by the same person with the journey history to exactly surround the revenue check would be accepted as mitigating circumstances.
If the passenger doesn't check their accounts for a few days/weeks it can still be sorted out at anytime up to a year later as that is how long contactless journey records are kept.
Most people I would reckon would only have one contactless card, or have say a debit and a credit card and know which one they always use (to ensure that they benefit from capping).
The benefit is that those who are chancing it will not get into a conversation at the time and those for whom a touching error means they are inadvertantly invalid won't be humiliated by the semi-public and usually lengthy conversation that then takes place.
