Afterburner is a bad idea because it goes against the whole point of the A-10 engines, which is efficiency. Afterburner is used on fast flying airframes, AB is used in those cases because it provides a very high exhaust gas velocity, as well as additional thrust without increasing the inlet diameter, which is great because no additional drag is created. But afterburner is terrible for efficiency, not only for fuel but also propulsive efficiency.
It's like traction on your car when the wheels spin too fast, you lose traction which results in a bad acceleration. It's more or less the same thing here, unless you're at a decent speed, using AB at slow speeds, which is what the A-10 was designed for, isn't very efficient.
The A-10 is a very draggy airframe, the maximum allowable airspeed is just 450 knots, you wouldn't want to go past that number. The idea behind the A-10 engines is to move a huge mass of airflow but at a slower speed. AB does the opposite, much lower mass flow, but at a very high speed. And according to the Kinetic energy formula, energy is equal to 0.5 * mass multiplied by the square of the speed. I.e, in order to shoot a little mass of airflow out of the exhaust at Mach 2, you need four times the energy.