As far as I can tell, the primary reasons to partition are a) preventing log / temporary files from growing without bound, and b) ease of restoring from backups (e.g. don't need to restore the stuff that came off your linux CD), or if your machine was compromised. You can physically go to the machine, wipe the OS paritions and reinstall from CD or restore from known good backups, while leaving the data partitions alone.
Also, partitioning is handy if you planned on keeping radically different sizes of data. If you were spooling a newsfeed fer instance, you'd want a filesystem with a whole bunch of inodes and a small block size. If you just had one or two big files (say a monster chunk of raw video), ten you'd want a much smaller number of inodes and a much bigger block size. Multiple partitions let you make seperate tweaked filesystems like that.
You can cause yourself all sorts of headaches if you go hog-wild with partitions, and discovered you planned incorrectly as one partition is bursting at the seams and the other is a vast empty landscape.
Personally on my rinky-dink little SuSE colocated box, I have a small /boot, then one big partition on one drive, and one partition on the next drive. Works pretty well for me