You could make a good tennis game on the DS with touchscreen controls. Basically the top screen would be the court and the bottom would be for what I'm about to explain. You move your charcater with the D-pad or the buttons depending what handed you are (you could set L/R for something, maybe just throwing the ball up in the air when serving, I dunno) and when you've moved your charcater in position, you make a movement on the touchscreen to hit the ball. You'd hit it at whatever angle you moved the stylus, and depending on the length of screen you cover, you get harder or weaker shots.
You could also add backspin or chip it or lob it etc. with different movements. For example, you move the stylus back rather than forwards to chip it. In fact, forget that L/R thing I said earlier, you could hold that down when doing movements with the stylus to change them. So for example if you moved the stylus back without holding L, you chip the ball. However, if you hold L while moving the stylus backwards, you lob it.
It could be awesome for a tennis game, I wish developers would wake up and start trying these ideas non-developers are coming up with for the touchscreen.
It would be kind of too big to fit in your pocket wouldn't it?
I think he means why not use the 2 screens to cover one court instead of splitting it with stats rather than questioning why the DS is built that way.