If you are liable to lose your keys, you are liable to lose the spare too, hence swapping. The idea being you might look after someone elses property better than your own.
I'd be quite concerned about taking a trip with people that are scared of looking after their own bike keys. What do they think is going to happen to them? Certainly wouldn't leave a set on the bike. What a pleasant surprise that would be for a thief! Right up there with leaving cash hidden on your bike. What a great way to encourage theft!
Each to their own and all that, just don't see the point. Worst case I could get the missus to post me the spares, but I'd have hot wired it long before they arrived.