A recent article on EETimes compares the Cortex A7 core to the Cortex A15, and reaches the conclusion that perhaps it would be better to use Cortex A7 in most, if not all situation, because it's a much more efficient chip. Cortex A7 takes about 1/5 of the space Cortex A15 takes inside a chip, and uses 5 times less power consumption, too, yet it's only about 2-3x less powerful. So why aren't we using Cortex A7 everywhere?
The answer is because the comparison is not entirely fair. Yes, we've seen TSMC experiment with Cortex A9 before and push it even to 3.1 Ghz, but Cortex A7 was never meant to reach such high clock speeds. It was built for low-end and mid-range phones, that can't afford to pay a lot on the chips, nor on the batteries. That means low-end phones can get good enough performance with Cortex A7 (around Cortex A8 level), and the phones using them don't need large batteries either, because the chip is very efficient, so it can achieve the same battery life as higher-end devices, but with smaller batteries. That means lower costs for the lower end phones.
Even if you doubled or tripled the clock speed of Cortex A7, you probably wouldn't get the performance of Cortex A15 either. Cortex A15 is a more advanced architecture, that handles certain types of instructions a lot better than a simpler chip design like the one in Cortex A7. That's why so far Cortex A15 can achieve higher performance with lower clock speeds even compared to the Qualcomm S800.
Also, a quad-core Cortex A7 may be faster for heavily multi-threaded apps, but as we all know most apps are heavily single-threaded. So even if in theory a quad core Cortex A7 can beat a single core Cortex A15, in reality that single core should have much faster performance for most apps. Plus, high-end phones today come with quad core Cortex A15 cores anyway, so Cortex A7 can't possibly match that.
All this being said, Cortex A7 is a great chip for the low-end, and this chip combined with Android Jelly Bean, should give a very decent experience even for sub $100 unlocked Android smartphones in emerging markets. It should be an even better match for phones arriving with Android 5.0, which is rumored to run better on lower-end devices.