Processor power use scales with frequency as well. See [0], specifically the 'Low Memory and Power Devices' section, which shows power savings running 4 cores at lower frequency versus 1 core at max frequency. I think they eventually decided that running at max frequency on all processors and sleeping wound up being most efficient but that's my memory and could be completely incorrect.

[0] http://blogs.s-osg.org/servo-adapting-c-to-work-on-the-web/

Getting work done faster lets you put the CPU to sleep faster.

In theory, more parallelism can give you a) more work done in the same time, b) same work done in less time, or c) same work done in the same time using less energy due to lower frequency.