A dualcore would suffice for any phone right now. But I also don't think that a quad core Cortex A9 would be too power hungry. By dynamically disabling cores that aren't needed on low-level usage, it shouldn't use any more power than a single core. Enabling them on the other hand, when more CPU resources are needed, allows a task to complete faster and therefore sending the CPU into deep sleep much earlier - which as you may know uses no power on this architecture.

Besides the CPU wouldn't have to work at the maximum speed when distributing tasks on more cores, thus enabling the CPU to use less power when running multiple cores at low voltage and low clock, than running just 1 core with maximum clock and voltage.

But it's true that the maximum power consumption CAN be four times that of a single core, when all four cores run at maximum clock and voltage. That means that if you are watching a HD movie, playing Quake and compressing a .zip archive at the same time, it will drain the battery much faster. BUT you wouldn't be able to do all of this on a single core anyway and it won't happen to often that somebody uses his phone that way .