Some kind of "update" on this issue:
Recently some Linux distributions are merging /bin into /usr/bin and related /lib into /usr/lib and sometimes also (/usr)/sbin to /usr/bin (Arch Linux)
So /usr is expected to be available at the same time as /.
The reason being that it is an unnecessary complexity.
The reason was once having only /bin available at boot, but having an initial ramdisk makes this obsolete.
I know of Fedora Linux (2011) and Arch Linux (2012) going this way and Solaris is doing this for a long time (> 15 years).