I have managed several extremely large fleets of computers and I can tell you they all use frequency scaling. I seriously doubt that your statement applies to "most server farms" when properly weighted.
It depends on who pays for power. When I was running a couple thousand machines on managed hosting, we disabled frequency scaling, because it wasn't our power bill and predictability and ease of measurement is nice. But at places where we paid for power, then frequency scaling was enabled (and sometimes they'd throttle things down to manage hot racks etc)