User blog comment:Xilinoc/Ch. 553 - Frozen Cross/@comment-1747976-20131020025317/@comment-5606268-20131021053032

I think the fact that Ichigo's bankai hadn't been stolen at the time was due to his Hollow side. I agree with Morph on this one, I also think the way Ayon was assimilated would be different to the 'stealing' of bankai. Aside from that there is Ichigo's Quincy side as well, which could be a factor.

From Urahara we've learnt that the Quincy, currently cannot absorb a hollowfied bankai, as we've seen the revival of two bankais as is, and that is a potential flaw or weakness for them.