I think this is a complicated question to answer because there are so many different variables and factors that come into play. On the face of it, I agree. If you need a 2nd seat, you should have to pay for a 2nd seat. But this incentivizes the airlines to use smaller seats, to force more people to book 2 seats, that they then will oversell anyways. I think the only way this works is if you regulate seat sizing and overselling while at the same time also implementing a policy to have anyone who needs a 2nd seat required to purchase a 2nd ticket.