Tugs are responsible for a lot of damage and pollution. The potential savings are substantial. There are numerous benefits.
It is kinder to the NG than existing methods. Much kinder than extended towing....
The electric motor industry has flat-hat tendencies.
As far as traction is concerned, it evidently worked. Main Engines used for taxiing are
hopefully running at a fraction of their potential yet unnecessarily gulp copious quantities of fuel. Actually less fuel would then need to be loaded as much less would be required to taxi out AND TAXI IN on landing.
A 737NG APU is orders of magnitude more efficient at generating power and capable of providing the required power for taxiing. The Chorus Motor under design & development is fit for purpose.
Most pilots would be thrilled to be in total control of their craft, gate to gate. Besides they do tend to be more caring than some tug drivers. Certainly more qualified. There are numerous 'incidents' every year in this sphere of operation. More potential for savings.
Main Engines would only be spooled up some minutes before takeoff (as per manufacturers requirements.) Similarly on landing. The previous 'Test Quote' modus operandi was designed to ensure that it is fit for purpose. It satisfied Boeing.