6 seconds a lap doesn't give enough benefit to combat the weight and aero issues it causes. The only advantage it's given teams is as a quick boost off some start lines and to help keep cars behind them.
My understanding of the rules pertaining to the systems was they could not provide any benefit until the car exceeds 100 KPH. Cars do not get any direct benefit at the 'start', they may be able to get an assist for that last push into turn 1.