I smell Modus coming!
Since I authored a Dissertation on Partitioning in the past (and working on a rewite/AnandTech FAQ), I will only say that I heavily rely on partitioning, as I am attempting to multi-boot SE7EN OSes on one 40GB drive. Why seven OSes? Because I can.
Forgot to add: Folks with one OS may not have much incentive to partition, other than for organization's sake.
-SUO