That's a case where initramfs is really usefull because you can do 
a lot of tricks in the initramfs (including an interactive session). If, 
then, you only mount /usr, /var and /home, you cannot consistently run 
package management tools, because they modify also files which aren't 
under these mountpoint, and modifications to your initramfs are lost at 
reboot.

     Therefore, when you are done with the sequence in initramfs, you 
must end up with a pivot-root to an nfsroot. This nfsroot is on the NFS 
server, typically all on the same partition (eg /srv) and there is no 
benefit at all in having a /usr in it, except this is what the packages 
expect.

     Didier