A lot of those designs predated initramfs. I have yet to hear of a setup where there's a good reason for not mounting /usr in the initramfs. (Though, yes, I have heard of setups where you can, it would just be a fairly involved change. It is a legitimate problem for these sysadmins to be compelled to rearchitect things, although I'd personally blame that more on the distro model where all parts of userspace are tied together than on systemd itself.)
