Comment 4 for bug 139057

the is a reason for having two hooks: one for initramfs and one for the init script.
the initramfs hook is only for mounting the root filesystem. So the only point in the patch above is when you have your root device across several crypted lvm or raid volumes on different hard drives.

I think having a patch from cryptdisks doing the passphrase caching would make more sense.

IOW: I'm not at all convinced by that patch above, I'm sorry.