half of motivation for 3D i see is the model of the brain, not the actual geometry of the brain, which is about 2D+ (don't remember the fractal dimension value of the brain), more a logical model of it where computational power comes from having number of connection an order of magnitude larger than number of connected neurons. Our current electronics has it backwards - number of connection is orders of magnitude less than number of connected elements. 3D seems like a way to build computational devices with increased number of connections (that though requires some work on software side as well to benefit from it)