Geez, such harsh critique you guys give. :lol:
If I'm not mistaken, this sounds a bit like a touch screen version of Cannon Fodder or similar game. Run in directions using the nub, fire in the direction of the tap.
The idea behind the game sounds fun, but I do agree with hansfranz on the point that "just because it's there doesn't mean you have to use it."
The idea sounds good, but the control scheme would be very uncomfortable. Try and position your hands where you feel they should be while playing. Were it me, my left hand would be holding the Pandora, gripping the device with the base of my thumb and middle, ring, and pinky fingers, while my index finger and thumb tip did the button pressing. My right hand would either be hovering above the touch screen with the stylus, or trying to hold the device while I switch from the nub to the d-pad.
While switching from D-pad to nub, or the inverse, my grip on the device would be significantly reduced, and dropage may occur, resulting in one very broken Pandora, depending on the height of the drop and whether I was using a wrist strap or not.
Also consider that when both hands are not gripping any controller, the usability of the hand holding it becomes reduced. I just tried holding my PSP with one hand and it doesn't work well, mainly because the back is flat. Xbox or PS controllers are ok because they have ergonomic handles that allow for easier 1-handed gripping.
My suggestions: if you really want to use the touch screen, try not to make the player have to switch between the D-pad and the nub during critical moments of gameplay. Map the weapon selection to the touch screen in case players don't want to reach down with the stylus hand to select weapons on the buttons.
Would I play it? Yes, probably. I say do a prototype of it, play with it, see how it works. Sounds like it could be fun.