This is significant for traditional microwaves, which might well be on for 10 seconds then off for 10. "Inverter" microwaves switch so fast that they are effectively reducing the power. The only thing where I've noticed this make a difference is in softening butter - it's less likely to end up with half of it still solid and half liquid.
(It's reported that at very low powers, inverter microwaves also use a several-second duty cycle, perhaps because they can't work efficiently otherwise.)
-- Richard