Power is not really the main motivation for multicore processing, it more has to do with performance for the tasks typical for a given system.
The big thing is that increasing clock speed only really helps if you have tasks that require huge blocks of computation. In this case, high frequency processors with deep pipelines really begin to pay off.
However, most tasks that run on a normal workstation need processing power only intermittently in response to events, but there are many hundreds of these tasks. In this case, it becomes advantageous to have better thread-level parallelism, which can be attained by having multiple processor cores.
The reason why multicore processors have really begun to take off in recent years is an effect Moore's law, not a cause of its continued validity. Chip producers can fit more an more transistors on a chip, but at this point, there's not a whole lot left to do to improve single core performance for typical workstation tasks. After all, adding more transistors doesn't really correspond to increased clock speed. So what do they do with all those transistors? Add more cores and increase multitasking performance.
It's not really a new phenomenon. Multiprocessor systems have been around for quite some time. It's just that now, with increased transistor counts, there's little reason to put cores on separate chips.
When you start talking about having dozens of cores, then significant power savings is possible, because you can turn off the cores you don't need when the system is under low load.