One of the problems is that for overseas international flights, one usually buys a return ticket - buying 2 one-way tickets can become very costly. So if one can enter just one country's passport information.

Now, if one goes from Canada to USA, one can buy 2 one-way tickets. So if one is a Canadian-US dual citizen, would one not preferentially enter the US passport information for the US bound flight, and the Canadian passport information for the Canadian bound flight?