Gotta agree with this one. The whole OS is small enough to stuff on the small expensive media now and just mount it at /usr if that is what floats yer boat. It is /var and /home where the massive consumption happens these days.
And I kinda like the notion of having all of the executables and libraries on /usr where you can just mount it readonly and have done with 90% of potential exploits... especially you mount /tmp as a separate filesystem and remove exec. Then you only need to watch out for /etc.