One use case I can see is large xen hosting providers. Upgrading the domU kernels is a real
pain for such providers, because just rebooting all the user's machines won't work; a certian
number will fail to come up, users will hate you. Instead, a good hosting provider will let
the user choose when to reboot into a fixed kernel. But then some percentage of users never
will, and instead get cracked.
So I can see hotpatching the kernel being really appealing in that situation, even if it's
annoying to manage.
(Personally, I'd rather my xen hosting provider just let me build and run my *own* kernel. But
that's just me.)