User blog comment:Xilinoc/Ch. 656 - GOD OF THUNDER/@comment-26544002-20151218052353/@comment-24769684-20151218112416

Bankai shouldn't be used immediately upon confronting an opponent, as it is only meant as a last resort. Ichigo had wrecked a couple Sternritter with just his Shikai, why would he fight Askin differently? The only time he kept Bankai active the whole time was in Hueco Mundo because in his fight with the former Espada Ichigo realized he wasn't strong enough to take on the Espada with just his Shikai. The Quincy are another story.

From a real-world, logistical standpoint, Kubo is not gonna reveal Ichigo's Bankai on Askin of all people. That would be a waste. We will see it when he fights Yhwach