No, we had even seen that a stolen bankai, might be performed in it's weakest or even weaker form than the original user have first invoked it. An example of that was when the Spear Quincy was only able to invoke the very first version of Sakechibe's bankai, what's more, at a weaker state than it was used before on Yamamoto.
The reason why Ichigo's bankai can't be stolen, is probably because the modern day Quincy techniques (like Oppie's Jail) doesn't work on Quincy. You wouldn't want your subordinates to fight between themselves over bankais, do you (then again switching medals might be possible ;P) ?
The reason why they didn't steal Yamamoto's bankai before Real Bach appeared, was because they were forbidden from doing it. Bach wanted his bankai for himself, since he was his nemesis and all that. What's more, we already have seen that giving a strong bankai to a weak Quincy, would be a waste of it's power and that Quincy would only be able to unleash a fraction of it's power, like the Spear Quincy.
At first, I thought that Unohana mastered healing methods to save lives, because she cherished life itself. But now, I think that she might have learned them so that she can keep alive her opponent for awhile longer, prolonging their fun at cutting each other.