Single core performance matters more for current apps and the current iOS.
I don't doubt that developers can think of ways to use extra cores, and once Apple implements some sort of window management, those cores will be used to the hilt.
iOS Apps are in a unique place since Apple rejects ones with bad performance. Traditionally developers have released slow apps and let hardware catch up, but they can't do that with iOS. As a result the apps lag behind hardware performance, as in, they don't exploit it until it's been out for a while.
It's great to see Apple finally add more cores to their ARM SoC. Now in a few years all iDevices will feature quad cores and then the real excitement starts on the app side of the equation.