Even with a single drive, I will partition it. I will set-up one for the
os, one or more for programs, and a smaller one for data. I like to do
this mostly for the smaller block size(forgot the correct terminology).
If a program is 'core' to the operation of the PC, I will install it
into the OS drive. Otherwise, it would get installed on a program drive.
This same thought can be applied to multiple hardware drives.
A lot of people won't do this because they don't like the hassle of
remembering to change the C: to D: when installing stuff. Then you got
the software out there that still can't quite understand that a PC can
have more than one drive letter.