I think it would save you someday, when there is nothing writing in /usr so the writing in /home would not cause much damage. On a system with a huge root partition, an incomplete writing might damage the whole filesystem.
Fsck would be faster. newfs (mkfs) would be faster. I found NetBSD spend so much time when it do newfs a 32G root partition (installing NetBSD in hyper-v).
Also for the /tmp partition, we can use memory filesystem (tmpfs) if we have 4G of RAM or more, instead of physical disk to store things that are cleaned on reboot.