For one player, you don't need the GBA. A 'SP' screen will just pop up on the screen. But for more than one player, that wouldn't work cause the screen would be blocked.
Am I the only person that perfers to play 1 player without the GBA? I have the original GBA, so I need a good light source, so I gotta adjust where I hold the thing... gotta look at the screen, then down on the GBA, then back up... just gets annoying.