That would be a good idea for older chips like the Pentium M, but Intel's latest CPUs feature power gates so efficient that the current draw of an idle CPU that

That would be a good idea for older chips like the Pentium M, but Intel's latest CPUs feature power gates so efficient that the current draw of an idle CPU that has been shut down, is very close to zero. Furthermore when all but one core is shut down, the clock speed of the remaining core can actually go UP by about 200mhz (called Turbo mode), providing a speed benefit to a single-threaded task.