AEDs are smart enough not to shock unless they detect VF. This is important because defibrillating a normally beating heart could actually cause VF. Would-be rescuers may not always be trained well enough to tell the difference between cardiac arrest and something else, such as a seizure or fainting.

But to your specific question: If a patient is in cardiac arrest but not in VF, then this means his heart has completely stopped. Forcing defibrillation won't likely improve the situation, but nor can it make it much worse.