Towing might make sense for wide bodies. The tug driver could also be qualified as an engineer (standard pracitice for some US airlines I believe). Crew costs are the same whatever the size of a/c. You could tow with the APU running or without but the latter would require removing air hoses & power lines so would be rather a hassle. The a/c would still be under the command of the flightcrew. There could be additional work for ATC unless extra roads were built - airports with close parrallels might require underpasses or additional ground controllers. (What would happen at FRA?)
Boeing (I'm not certain about Airbus) are not happy about stresses on nosegears. New a/c could come with strengthened gear as standard. The cost of strengthening and recertifying existing nosegear would doubtless be prohibitive.
I reckon that the savings would be worthwhile at airports with long taxis such as JFK, FRA & LHR.
Electric towing makes more sense for lighter aircraft. You might need to fit a larger APU but that could probbaly be justified. An article in Flight this week quotes savings of 2 - 4%. This would obviously vary by route. For a sector such as LHR - AMS it is possible to spend as long taxiing as flying so the savings are obvious. I've flown between JFK & DCA where the published sector time was three times the actual flight time. Much of this is down to very long taxi times at JFK.
The flight article notes that the nose wheel only carries c10% of the mass of an a/c. This raises the question of why not fit motors to the main gear - most cars are rear wheel drive, or are there more serious technical issues with this?