Some relevant points I think worth considering here are:
1.) Multi-core processors don't do anything for performance unless the software that runs on them is optimized to use multiple cores. Even on desktop computers, most apps still aren't optimized to benefit from multiple cores. The OS's are, so the computers generally handle multiple threads better and perform faster. But until a version of Windows Phone OS is released that is designed to use multi-core processors (WP8 I'm sure will be) there won't be any advantage. Unless the tame sort of tombstoning multi-tasking that WP does now changes to something more robust, I think dual cores aren't going to do much. Remember it will also take a while for native WP8 apps to come out, and only SOME of those are going to be designed to actually use multiple cores and benefit from them. Multiple cores aren't like adding clock cycles, they don't automatically make everything run faster all on their own.
2.) More cores don't necessarily use more energy. Samsung's new quad-core Exynos chip actually uses significantly LESS energy than the dual-core chip it supersedes.
3.) There's no sense is getting too excited about dual-core WP8 phones now. Even if they start shipping them at the end of the year, they aren't going to be amazingly different from the single core processors until brand new apps are written to use them. Even then, you are only going to see a difference in apps that demand a lot of performance. Like games for example. Most of the apps you use every day won't run any faster with 2 cores. There's a lot of marketing hype behind this. The most exciting part about dual-cores is not an overnight performance jump, it's what it will mean for future development of the OS and apps much further down the road, say a year, two years down the road. It will enable more robust features, like better integration of voice recognition features throughout the OS and much better, more complex games with better graphics. But it's going to take a while until we see those things.