If a person actually takes up two seats, under any seating scheme, a case can be made for charging the person extra, since the individual is depriving the airline of income, and another would-be passenger of access to the flight. But this situation does not arise often, and this example seems to get raised to justify the entire category of making weight-based cost distinctions, rather than to save the airlines money or expand the number of available seats. Flying is stressful and expensive. Airlines should provide better service to remain profitable.