Heard from my friends flying the A320 with CFM engines, you need most of the time to had some power to taxi with both engines running. I guess it can easily be an hassle to do single engine taxi out? I only did once in Beijing single engine taxi out on A321. But with IAE engines even on single engine, you barely need to add power. A light A319 will even accelerate above 30 kt with one engine on idle power.