Once you start playing with Parallels images, ripping DVDs, and maintaining a Windows partition, even the 100 GB drive in my MacBook Pro can feel tight, hence my tip on reducing OS X and application disk space. But even if you take those measures (I clearly have) and still max out your disk, what happens when OS X runs out of space? You might see a dialog like this:
I always find it interesting what a piece of software does when it hits a boundary condition. In my mind, this is what sets the good software apart from the bad. Good software gracefully degrades or fails when it encounters a boundary condition, bad software chokes and gives up for dead. Even good software can choke and give up for dead, but the situations under which it does better be extremely rare.
This Disk Full Alert dialog (with double exclamations) is amusing to me because it's one of two times (that I know about) the OS X end user is even aware that their hard drive might be used for memory. It's only really at the breaking point for the operating system virtual memory subsystem that the user is made aware of it, and this is really the way it ought to be.
By the way, the other time the user might be aware of the virtual memory system is by setting the Secure Virtual Memory preference in the Security System Preferences. I really haven't found a reason this should be an option at all. Apple should just turn it on and take the preference away from the UI. I can see no performance difference all they way down to an iBook G4 933 MHz, why isn't it just on by default?