First off, no question is dumb. If it is intelligent, that is, and yours certainly fits the bill.:thumbup:
The engines take time to spool up and down. Idle to full power may take about 4-5 seconds.(In new engines, old ones take longer.) This is a LONG time if you find yourself low on short final over an obstacle.:joystick: However, the relationship between delay length and engine speed is not linear. That is, at about 75% core RPM, the time to full power falls markedly. The speedbrakes are extended to allow the engines to be run at about 75%-80% RPM (just a guess, but probably close) on final. If you need to go around, you can remove all the drag of the brakes in about 3 seconds. Combine this with the engines already putting out about 80% thrust, and it's easier and safer to go around. The alternative would be to run the engines at or near idle, and in that case, you would need to predict every power change by at least 4~8 seconds.
One more effect to keep in mind is that rapid throttle changes can cause a flameout. For example, being low on final realizing: ZOMG I NEED POWER NAO!!!!!! and slamming the throttle to the stop can result in a total loss of power, dignity, money, and possibly life. Of course newer engines with more sophisticated controls eliminate this problem. This is the case on the A-10, so you can probably slam the levers around without too much worry. However, that's bad karma and is best avoided.
Just my US$0.02