...But sometimes only because it has to - because the logistics chain cannot be expected to provide the artillery with a supply of ballistically genuine blanks.
In any case, what reason do we have to think that it would be a bad idea to fire full power shots into empty space? We have never really heard that this would result in a "shortage of ammo" as such. We do hear that phasers can be "depleted", but there is no evidence that this would be as the result of firing high-power shots.
Supposedy phasers get their energy from the general power grid of the ship, and can be fired with as little as what's in the batteries, not even requiring auxiliary let alone primary power. Thus, when phasers are said to be at as low as 4% at the end of the ST:NEM battle, but the ship still retains enough energy for impulse maneuvering, it is difficult to believe that this phaser "depletion" would be as the result of energy wasted through high power shots. Energy should still be there aplenty for a thousand or ten thousand additional high power shots - something else must be depleting the phasers.
Okay, so perhaps the depletion does come from high power shots: perhaps they cause such stress to the phaser emitters that these degrade and cannot easily be restored. But low power shots might have the same effect. And ramping between low and high power might be the worst possible thing to emitter lifetime.