James Bruce from ARM explains that, in the tablet world, there are no use case for 8 core SoCs. However 8 is a lucky number for Chinese customers. The Exynos 5 octa is based on 4 cortex A15 combined with 4 A7 for instance. According to James Bruce 75% of the time, such a processor is either asleep or waiting for interrupts. So the actual need for the processing power is not proven.
Out of the 250 millions tablets shipped in 2013, 100 millions were shipped with a Chinese SoC inside.