When I first heard about this on the news, it was being billed as a cost saving exercise, which would also save on emissions . My worry would be that you get to the runway, they go through preflight checks & or start the engines and discover there is a fault, and the tug then drags the plane back to the gate. I can see it as a sensible thing to do on landing, especially on early morning late & evening arrivals, but I'd be a bit pissed off if something minor that could have been sorted at the gate before pushing back and reaching a remoter area of the airport, caused an extra delay.