Nice idea, but is it practical?
Jet engines, even at idle, burn fuel (and produce residual thrust, which is what keeps the aircraft rolling).
So, unless the engines are only started at the end of the taxying phase (and would this be allowed, possibly not) how much fuel will actually be saved by using this system?
It's also a bigger logistical problem if the aircraft goes tech on start when it's already in a queue on the taxiway, as opposed to on stand. There is probably nowhere to turn off except via the runway. Many tech issues are discovered during, or just after engine start.
Where will the power to drive the electric motors come from? Additional batteries, a bigger APU, or a generator (i.e engine driven...see above)?