An interesting topic. I have been comparing recent aviation ID checks with other forms of international travel. On Brittany Ferries you show your passport to an official on departure and arrival, same with the train, which permits a visual ID check and passport scan if necessary. You could, in theory, swap passports after that point but would be in trouble on arrival because your passport picture and you would not match unless a substitution had taken place after check-in. Most people keep a firm grip on their passport or other ID which I think is the best protection against abuse. Having said that, though, I am aware that some people carry multiple passports. Would it matter if they showed different passports at check-in and arrival?