For low power chips, what advantages would there be for stacking? My thought is that low power chips probably aren't going for performance (since they are low power), so stacking for performance wouldn't really be buying them anything necessary. Maybe you could just get smaller form-factor low power chips.
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)