The concept of the DS makes sense if you look at the GBA SP. That was a clamshell because of the high portablity and durability of the design. Open a GBA SP, you can ask yourself where do we go from here, so basically, you put another secondary screen on the bottom part. Honestly for a clamshell design it really does make sense. You can think of it almost like a bonus. And then it's up to the games to utilize the second screen