Why is defibrillation in asystole ("flat line") useless?
In layman's terms: an Asystole is not affected by the electric shocks of a defibrillator. A defibrillator is used when the heart goes in fibrillations because it actually CAUSES an asystole. The idea is that you basically reset the heart to a blank state so you can start normal CPR procedure to help the heart go back to a normal rythm. That's why you see them applying CPR between shocks.

sources:

http://emedicine.medscape.com/article/757257-treatment

http://en.wikipedia.org/wiki/Defibrillation#In_popular_culture (citation needed)

http://calsprogram.org/manual/volume3/section12/CV/08-CV7AsystoleTreatment13.html