Quoting John Hughes (john@???):

> The separation of / and /usr is a relic of really, really tiny disk sizes.

It _originated_ in everything not fitting on one disk on Ken Thompson
and Dennis Ritchie's PDP-11, at a point in 1971, originally as a place
for user home directories. Rob Landley has a good version of the story:
http://lists.busybox.net/pipermail/busybox/2010-December/074114.html

However, there are other compelling use-cases for it. (The
freedesktop.org kiddies claiming those use-cases don't exist or
shouldn't matter doesn't signify.) As to 'just use an initramfs for
that', I'd personally prefer my servers not have them, towards the goal
of simplifying system architecture.

(Above is not an argument of any sort about distro policy. Distros
are welcome to set policies according to their criteria. I will then
ignore and override those policies to run my systems as I prefer.)