Let's say it's you and your wife traveling together. You have a frequent flyer mile number and she does not. Can you use your frequent flyer mile number for both tickets or can it only be used for its original owner? Or maybe only once per flight or some such?
In some ways this sounds unethical but in the case of a married couple I think there's some pragmatism to it as well. Let's say 25k will get you a round trip flight to anywhere in the US. You and your wife each have 12.5k so if you want to get a free round trip you'd have to pay to transfer the frequent flier miles to one or the other whereas if only one account got them for both people then you could spend them at no inconvenience to yourself.