It's simple on the 320. You can shut down the engine right away provided you haven't used more than idle reverse. As stated by others, it saves fuel and brake wear and the speed is much easier to control. On a long taxy (10 mins) it is possible to save about 80kgs of fuel which multiplied by each trip per year saves a lot of fuel and CO2 emissions. Airbus insists that the APU generator be on line before shutting down No2. This bit I could never understand. Must be afraid of problems on one generator unlike Boeing where it's not a problem. Now, starting one engine while taxying out is another question!