If you get to try a UNIX system (Mac OS X, Linux, etc.), you'll see that they don't use drive letters for access. Instead, everything appears under a single virtual file system. For example, this Linux netbook I'm using now has two solid state hard drives; at the low level, they are different devices (/dev/sda and /dev/sdb), and each has one partition (/dev/sda1 and /dev/sdb1). The first one contains the OS itself, and is mounted as / (the root): the second one contains all user data and is mounted as /home (i.e "under" /).
If I only had one drive & partition, /home would just be another directory on it, but the point is that either way it's always consistent to applications and users. One advantage is that I can completely wipe the first drive and reinstall Linux on it, without touching the user data on the second drive. I'll be putting Linux on another 1-drive system soon, and so I'll be setting up a separate partition for /home for that reason.
I wish that Windows had that kind of option more easily available, but the security system they use would play havoc when it comes to reinstallation. I know it can be done afterwards with mount points, but not during installation.
Last edited by bnt; 24th Mar 2012 at 11:16.