In that it can't be reused while onlookers who have already seen it are watching. Not sure if this has been suggested before, but I just thought I'd bring it up.
He can use it's abilities without calling out it's name again on those who have already seen it. So I'll suggest that, much like Ichigo calling out Zangetsu's name again while fighting pre-Bankai, Ichigo has been immune to the use of Aizen's shikai thus far due to the presence of those who have already seen it.
And Ichigo needs something to keep Aizen from just going out and calling it's name the second he shows up to save them.
I'll also go out on a limb and suggest he won't be able to use it if he goes into Bankai, assuming his bankai is an entirely different asset.