The undercarriage wouldn't even notice the forces. Compared to landing, the sideloads from taxiing around with asymmetric power would be trivial. You can land the Boeings with all of the drift intact at their max crosswind....that would be 15 plus degrees.
I guess savings add up. If an engine is burning around 600 kgs of fuel per hour at idle, that's 10 kgs per minute. If you get to the gate inside the cool down time, there will be no saving at all, but a 10 minute wander around London you could shut down one, and save 50 kgs. If you have 100 aircraft, and they were all able to save that 50 kgs, just once per day, then it would add up to 1.8 million kgs in a year..which is certainly not an amount of money to be sneezed at.
Plenty of airlines shut down engines. It's not unusual at all (especially at places where the drive to the runway can exceed the length of the flight (JFK)). But, I'm not convinced that it's done with the big twins (we certainly don't do it with ours, but that doesn't mean others won't come to different conclusions). The quads are a different matter entirely (well some of them anyway).
