User blog comment:Xilinoc/Ch. 552 - The Fundamental Virulence/@comment-5551986-20131008165851

I doubt a Visored's bankai cannot be stolen. Their bankai's or, their swords rather unlike Ichigo's do not possess the power of a hollow, but their own souls do. Urahara spoke about "adding a smidge of hollow reiatsu to a 'Bankai' which would make it poisonous, I also beleived he said "hollowfy a bankai". Unless when they become visored their swords also take on a more hollow like effect.

Also I have a little theory. The reason why Uryu was never able to become poisoned by hollows and also why pure blooded Quincy let mixed blood fight hollows first is that, mixed blood quincies would be closer related to humans, thus being closer to hollows, Humans being the opposite and turning into them at some point. My theory is that mixed blood quincies are immune to hollow poisoning, also aside from the Kurosaki's it would give reason as to why we didn't know about it before, as far as we are concerned Uryu and the Kurosaki's are the last group of mixed blood quincies left.