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The DS's buttons on the right side are not as egronomic as the original GBA, or the SP. You'll find your thumb traveling too far to the right, just over your right index finger knuckle.

Try to move your thumb in that direction without holding a DS and you'll experience a little discomfort. Now imagine that feeling for 30 minutes at a time.

Thankfully, Ninty have alread announced a design rework, I would imagine this being one of the reasons, and well, since I'm right handed, most touch enabled games don't use the ab/xy buttons.