Advice wanted on disk partitions

[Warren Jones]

We have a single hard disk on our system (an RS/6000 under AIX 3.1).
Can anyone explain the rational for dividing this disk into several
partitions, and then re-joining them (via mounts) into a single
directory structure?

I believe that earlier versions of Unix had 16 bit inode numbers,
resulting in a limit of 64K files/file system.  Thus if you wanted
more than 64K files, you would have to mount multiple partitions.
But AIX 3.1 has 32 bit inode numbers, so this rational does not
apply.  (I suspect 32 bit inode numbers are the norm for current
Unix implementations -- how about your system?)

As long as there is only a single physical disk, it seems more
efficient to configure it as a single partition.  This produces
a single pool of free space.  Otherwise, you could run out of
space (for example) in /tmp on the root partition, even though
there was still plenty of space in /usr/tmp (on the /usr partition).

Am I missing something?  Comments, advice and explanations are welcome
before I take the major step of re-configuring our disk.