It makes their job too easy, no challenge, as it's not on the verge of knock all the time.
Seriously, the only reason not to run it is the extra fuel it requires, larger fuel system, less k's to the tank. The positives outweigh the woeful economy tenfold. Clean engine, more torque/power, lower temps, cheaper, environmentally friendly, not having to line the pockets of thieving oil companies...