Hi Paul:
What do you understand for 'rebuild' a system? Do you mean backup data, rebuild partitions and restore data? Fresh install? Rebuild using same system moving partitions while is up&running?
If your user data is on a separate partition, it makes it -much- easier to perform a clean install (either of the current or a new OS version) on the 'os' partitions without worrying about the integrity of your user data safe on it's own partition.
A long time ago I used to install Suns with a reasonably complex arrangement of partitions (separate /var to prevent logs from filling up / and crashing the system, separate /usr mounted ro for extra security against hackers, separate /tmp (pre tmpfs), ...) but it seems to me now that the more complex your partitioning, the more likely they are to give you trouble.
For example, when sun and their sw vendors started providing software that installs in /opt (which was part of /) it caused a big hassle (/ didn't have room for giant software packages, /usr did!) involving convincing various installation tools to ignore that it's installation directories were now symlinks. Also, in general, the more you break up your filesystem, the more likely you are to have plenty of free space on one partition when another fills up unexpectedly.
Now I use just /, /usr/data and /boot (and swap) and life has generally been easier.
