I used my front shifter (left) for my clutch, but I have the quick fire type so I am not sure how the grip shifter type would work for that. And I am getting ready to put the throttle on my back shifter(right) to have thumb type throttle. So I would imagine a grip shifter on the accelerator would give a more traditional type throttle set up.
I origianlly did not want to modify my bike much, bolt on the motor and go, and be able to take the motor off and have my mountain bike back if I did not like the motor, that changed after the first ride, the motor is never coming off... But swapping cables out on your shifters is not permanant so if you don't like it, put it all back, I have not used my shifters once since adding the motor, it is set on 3 or 4 on the rear and that is the perfect gear for starting up, and for the one hill I sometimes have to peddle assist up.
Also by using your shifters, you will keep your handle bars a little cleaner and the controls less confusing.