Fair point on the resting on your chest thing. But kids are not going blind from playing 3DS. Why on earth would you believe such utter nonsense?
People going on about kids going blind from current 3DS, you can disable 3D entirely from the system settings so that they slider does nothing and kids need to enter a pin to switch on 3D.
What on earth are you talking about?
If you're a parent that is in any way responsible you use the parental controls and disable the 3D.
Also, if the headphone jack was on the top, the wire ups hang down in front of the screen, or behind where your hands are. Having it on the bottom means it stays out of the way.
Headphone jack on the bottom is a nuisance if you're lying down in bed/sofa as you can't rest it on your chest.