Seriously though, casual fans that Sony needs to make Spider-Man III successful will not go see it if Harry Osbourne is not promoted as Green Goblin II. The average cinema goer wants to see things tied up neatly. If GG2 is delayed until Spider-Man IV (without Maguire, Raimi and Dunst), Sony can kiss the casual viewers of Spider-Man goodbye.
Batman Returns worked because the bad guys, Penguin and Schrek were being chased by essentially two good guys, Batman and Catwoman.
Spider-Man III could work with two bad guys if they paint Harry as a confused tweener and Church as whoever getting disposed fairly early.