I think what it means is only that distros shouldn't assume that an installation has sole ownership of
/usr, not that everything in
/usr is expected to work with all FHS-compliant systems. I think I have heard of
/usr being served over the network (via NFS for example) for a bunch of systems running the same distro. Since
/usr is where the bulk of all installed files reside, this makes for a lot of space savings. Also, I think it's not unusual to have
/usr a separate filesystem in any case, mounted read-only for additional security, so the "must not be written to" part helps with that as well.
/etc can't be shared in this manner - some files, like
/etc/hostname are necessarily different for each host (though most files in
/etc can be so shared, I think). Nor can
/var - it wouldn't make sense to have two services on different systems logging to the same file, for example.