The way I understand it, the 's' in "/sbin" does not stand for "system", but for "superuser" - essential system tools which need superuser privileges (insmod, ifconfig, fsck, ...)
/bin should contain all the non-privileged essential tools for system to boot up and execute all its initscripts - stuff like echo, cat, grep, awk, sed, chmod and many other everyday tools.
In addition, it should contain tools which will allow you to repair a broken system in minimal environment (where only / is available to you). So, stuff like ls, mount, gunzip, dmesg, ...
/bin should contain all the non-privileged essential tools for system to boot up and execute all its initscripts - stuff like echo, cat, grep, awk, sed, chmod and many other everyday tools. In addition, it should contain tools which will allow you to repair a broken system in minimal environment (where only / is available to you). So, stuff like ls, mount, gunzip, dmesg, ...