The other half of the story for the efficiency cores seems to be the battery life. Most of these non-urgent background tasks are put on efficiency cores not only to separate them from my applications, but also to enable this insanely good battery life.
There is also a thermal benefit. Using the high efficiency cores puts a lower load on the cooling system. So even if the computer is plugged in and the performance cores are idle it may be best not to use them and let the cooling system gain some headroom for when the performance cores are needed.
