More computational power in what is essentially a pocket computer is never 'ridiculous'.
When you consider the amount of things you can do on the go with enough power, then you would probably think differently.
Of course, the size of the displays is a limiting factor, but then again this is consumer grade technology and nowhere near of our real technical capabilities (by contrast, this would be something out of the 'stone age' compared to what we could do by using latest science, advanced synthetic materials and state of the art methods of production).
Anyway, I don't see the point in having two sets of 4 core cpu's in a hardware that will all of the time behave like a 4 core one.
If its for power conservation, then simply set the cores to automatically lower the voltage and speed of the A15 chip down to the lowest possible level and have applications use specific amounts of performance/power for optimal results when needed, unless they are specifically written to take advantage of the full power of the chip (otherwise its just sloppy written program)