I notice that nobody has mentioned the obvious issue - money. Smart businesses these days understand that pollution= waste = money.
The idea of operational towing is a case in point. Mr Automan suggests his company is burning 12000Kg per day in taxiing. The B747 burn is approximately 1.5 tonnes per hour. 12000 x 365 = 4.3 million kgs annually.
What is the current cost of Jet Fuel? Simply do the numbers for the projected fuel savings, cost of the tugs and associated maintenance and operational costs, plug them into a discounted cash flow spreadsheet and you will have your answer.