These seem like orthogonal elements of a HA systems approach.
You seem to equate "there are two boxes and some HA software" with 100% availability, but
obviously it doesn't really deliver that, otherwise the phrase "belt, braces and skyhook"
Suppose you have a two box HA system, and one day you take down box A to upgrade the kernel.
But just short while into this procedure a tiny capacitor blows up inside box B, and the smoke
leaks out. Now you have zero boxes, and your HA solution has failed.
If, in contrast, you had moved the workload from box A to box B not so that you could reboot
box A into a new kernel, but just so that it could be patched using ksplice while quiescent,
then you'd survive this disaster with only mild inconvenience (box B having failed, box A
would suddenly become very busy, most likely causing your ksplice to fail)
Availability is a trade, usually against some combination of hardware investment,
administrative overheads and system performance. For some people ksplice could let them spend
a bit less money and get the same availability.