[EVO 4G General] wolrah:
Here's my thoughts:

1. Multiple cores are awesome. No multitasking system should ever be single core when multi-core processors are available. Even if you don't multitask yourself, the fact that the OS and your current foreground app do not need to share a core is a huge gain.

If you remember when dual core processors first started hitting the mid-range PC market it was amazing in terms of desktop experience. Those concerned purely with single-task bang for the buck like hardcore gamers tended to choose a slightly faster single core at the same price, but if you had a multithreaded load or multitasked while under heavy load multi-core processors were the obvious choice.

2. If you do an idle to death or full-load benchmark to death battery test, almost certainly a single core device will beat a multi-core device with otherwise equal hardware. Real world usage however is completely different. It takes quite a few CPU cycles to switch between tasks, and multitasking on a single core machine is really just faking it by switching tasks rapidly enough that you don't notice. If you have two or more cores, you waste less cycles switching tasks and thus get the job done faster. This means you can put the processor (or core, or section) to a low power state faster. In real world use laptops with multi-core CPUs, more RAM, and faster hard drives often beat their lower-specced counterparts on battery life for this reason, so it's not a huge stretch to expect similar results on a handheld device.

3. We don't have to choose between faster/multi-core hardware and Android improvements/bugfixes. It's not like Google's engineers are wasting a ton of time over at Qualcomm or any of the other CPU vendors helping them develop the new CPUs, the CPU guys are doing that themselves because it's in their interest and there's a demand. Software and hardware are developed alongside each other.

4. If you haven't had it and likely haven't used it, how do you know you don't want it? Looking back to #1, I recall back in '06 when myself and a few friends upgraded to dual core processors. I had some real convincing to do to one of my friends because he'd look at the gaming benchmarks and see that in most of the games he played he'd only gain 3-4 FPS (same clock speed as the single core processors we already had) versus the 10-15 he could gain from a faster single core at the same price. Eventually he relented and was converted by how smooth it made his system. The user experience gain on multi-core CPUs for desktops and laptops was great, I expect it to be similar on mobile devices.

That all said, if you don't see a gain for your use, don't upgrade. It's as simple as that. You don't need the latest and greatest phone, anyone with a fully rooted phone can generally expect to get all the new developments through third party ROMs like CM until their device is old/ignored enough to not be worth developing for anymore. Personally I'm envious of those with 45nm processors due to battery life and speed improvements, but I'll probably still sit back and watch for the first generation of dual core phones just to see where things go and then buy in when the second generation hit the market