> Multithreading does not make code more efficient. It still takes the same amount of work and power (slightly more).

In addition to my sibling comments I would like to point out that multithreading quite often can save power. Typically the power consumption of an all core load is within 2x the power consumption of a single core load, while being many times faster assuming your task parallelizes well. This makes sense b/c a fully loaded cpu core still needs all the L3 cache mechanisms, all the DRAM controller mechanisms, etc to run at full speed. A fully idle system on the other hand can consume very little power if it idles well(which admittedly many cpus do not idle on low power).

Edit:

I would also add that if your system is running a single threaded database, and a single threaded web server, that still leaves over a hundred of underutilized cores on many modern server class cpus.