The main historical reason why some root level directories were moved to /usr (which was originally the user directory like today's /home) is that Thompson and Ritchie's first hard disk for the operating system was full. The rest seems to be mostly retcons and backronyms.
At one point I stumbled onto a document from circa 1990 laying out a fairly coherent rationale - / was always a filesystem on the local machine while /usr could be a site-wide NFS share across many machines. Thus you got things like splitting off architecture-independent /usr/share from binary /usr/lib (because if you have a few different kinds of workstations, you'd want one /usr per architecture but /usr/share could be site-wide) and creating /sbin from binaries that had been in /etc from day one. Oh and they came up with /var (also possibly a network share) so /usr could be mounted read-only.
An interesting idea, but I have to figure the majority of Unix sites kept /usr on the local machine like always.
source: http://lists.busybox.net/pipermail/busybox/2010-December/074...