+1 dolog: ha valaki bejut a gépre, de nem root, akkor nem tud a ro mountolt partíciókba/logikai kötetekbe betenni dolgokat, átírni dolgokat, stb.
Különösen, ha pl. az nem is az adott gépen van; akkor még akár root-ként is bejuthat. Pl. egy read-only nfs-en levő /usr:
Example: /usr Network Share
With the merged /usr directory we can offer a read-only export of the vendor supplied OS to the network, which will contain almost the entire operating system resources. The client hosts will then only need a minimal host-specific root filesystem with symlinks pointing into the shared /usr filesystem. From a maintenance perspective this is the first time where sharing the operating system over the network starts to make sense. Without the merged /usr directory (like in traditional Linux) we can only share parts of the OS at a time, but not the core components of it that are located in the root file system. The host-specific root filesystem hence traditionally needs to be much larger, cannot be shared among client hosts and needs to be updated individually and often. Vendor-supplied OS resources traditionally ended up "leaking" into the host-specific root file system.
(Forrás: http://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/)
BlackY