I am a strong supporter of smart and clever use of partitioning. There are numerous reasons for this, but one of the chief ones is that, quite simply, putting all of your "eggs in one basket" is inviting disaster.
It makes much more sense to divide a HDD into a partition for the operating system, and a partition for user data (like documents, mp3s, etc...) Here is an example of why:
My machine has suddenly become unbootable, and I need to format the HDD and start again. But something is holding me back: I have user data files, which are important, scatter all over the place saved in the "Microsoft Office" directory, etc, and in folders called "Phantazmm", etc... Before I can format I have the tedious task of trying to recover this data.
Now if I had my data on a seperate partition, I don't need to worry about this and can just format and go. End of story.
It also simplifies the process of backups if all I have to do is simply backup the entire contents of my data drive, rather than pick out individual folders all over the place.
There are many, many other reasons why a dedicated data partition is a good idea, I will discuss more if you want. Certainly when I started at my current job they loved this idea so much that its become the standard now for machines across the faculty.