A cup game did come with the season ticket once upon a time.
I see your point though albion59, Blooze had the same issue a couple of seasons back. It was cheaper to go and see them if you paid per game, so you get to see all games, get it interest free (for those that pay for s/t on credit), and get to stagger the payments.
Everyone who sits in the same stand should expect to pay the same as the person next door. Why should you be penalised for paying up front? Aswell as guaranteeing you a ticket for every game, weren't season tickets introduced as a bit of a loyalty scheme?