Marketing departments like to latch on to a single number (clock speed, number of megapixels, etc) and use this to promote a "bigger = better" message. It's never as simple as that in reality, but simplistic messages sell better than complicated truths, and I'm afraid "core counting" has been one of these things in recent years.
The biggest truth that the "number of cores" metric hides is that not all cores are equal, indeed different cores can be very different in processing power, energy consumption or both. The next is that how the firmware and OS use the cores makes a huge difference (e.g. how efficiently tasks are allocated to cores, when cores are shut down), so different SoCs using the same core architecture and same number of cores need not be equivalent. But as said above, a properly-designed chipset+firmware will shut down cores when they are not needed, so they don't automatically result in greater power drain when not in use.
The real trend in recent years has been towards phones that idle efficiently but will use power heavily when pushed, so the variation in battery life with usage gets wider.
And this year it's perfectly possible to buy "mere" 4 core phones: Qualcomm's Snapdragon 820, used by almost all flagships this year, is a quad-core SoC (4 cores of the same architecture, but 2 with a higher maximum clock speed than the other 2).
