User blog comment:Bobbs3k/Can Ichigo's New Bankai be stolen?/@comment-5551986-20130630190501

I doubt it. We don't know how the Quincy even capture the bankai. I just can't picture Ichigo losing it. If he does lose it, it would be cool seeing him develop as a character without it, having to learn Kido, Hakuda, Shunpo and Zanjutsu techniques to compensate for the loss of his bankai. But I'm pretty sure in the upcoming chapters Oetsu, Mayuri and Urahara will explain everything.

If I had a guess, judging by what Asguiaro said in the incantation about shape etc, the device seemingly amplifies a Quincies ability to use Sklaverei without the need for Volstandig, it uses the medallion as a medium and a container, so they can access it whenever. This method could somewhat explain how a medalized bankai becomes less powerful. That is of course speculation. But its the only reason I can think of right now.