Reasons why you still might want to keep / and /usr isolated:
1. NFS mounts. Your local or initial BOOTP image has a minimal root, your (non-root-writeable, BTW) NFS /usr has Other Stuff.
2. Mount options. Root is often (and perhaps still must be -- /etc/mtab for example -- I've stopped closely tracking discussion to make root fully read-only) writeable. It may also require other mount permissions, including device files and suid. Other partitions don't require these permissions, and there is some Principle of Least Privilege benefit to mounting such partitions without them. /usr requires suid, but not dev, and may be nonwriteable except for OS updates.
3. Recovery partition. I'll frequently stash a second root partition, not typically mounted. It has minimal tools, but enough to bootstrap the system if the primary is hosed for whatever reason (more often myself than any other). Without a clean / /usr split, this becomes more complicated.
mtab is not a problem anymore. On Arch, /etc/mtab is just a symlink to /proc/self/mounts.
As for the recovery partition, you don't need the split for that, either. Just have a live system on the recovery partition that mounts the normal root FS. Then you can chroot into there for recovery tasks.
Right, that seemed to be the mtab solution Debian were angling toward. I think there were some odd edge cases where it didn't behave well, though I don't recall what those were. Perhaps the ability to specifically edit the contents to allow fixing of fubared mounts -- almost certainly loopback or NFS, both of which get quite twitchy at times.
I don't recall my precise thinking on a clean root vs. /usr split on the recovery partition, though it may have avoided some confusion over binaries. Or perhaps that you could mount the /usr partition itself independently if you wanted, assuming primary root was hosed.
Not being able to mount a separate /usr would negate that option.
Bind-mounting does give you some options. Still doesn't help if root's hosed.
And it wasn't an option when I'd first come up with this clever scheme.
One of my current challenges with Linux is identifying which information/education of mine is wholly outdated. This will happen to you in time as well....
1. NFS mounts. Your local or initial BOOTP image has a minimal root, your (non-root-writeable, BTW) NFS /usr has Other Stuff.
2. Mount options. Root is often (and perhaps still must be -- /etc/mtab for example -- I've stopped closely tracking discussion to make root fully read-only) writeable. It may also require other mount permissions, including device files and suid. Other partitions don't require these permissions, and there is some Principle of Least Privilege benefit to mounting such partitions without them. /usr requires suid, but not dev, and may be nonwriteable except for OS updates.
3. Recovery partition. I'll frequently stash a second root partition, not typically mounted. It has minimal tools, but enough to bootstrap the system if the primary is hosed for whatever reason (more often myself than any other). Without a clean / /usr split, this becomes more complicated.