I don't know if this is an issue with the A320 series, but I know that on some Boeing types (depending on the ambient conditions), with both engines running at ground idle, you'll sometimes need to drag the brakes to keep your taxi speed down. Hard on the brakes (and sometimes noisy in the cabin - upsetting the SLF). There is an obvious advantage to single engine taxi when this happens.
That being said, as an engine guy, I want to see the engines nice and warm before setting TO thrust...