Good question:
I only used to own 5,400. I recently bought a 7,200. Im glad I did. I do a fair amount of large file transfers (read: the p0rn). Im able to get large files over much faster. And when your doing that a lot, it saves you a lot of time.
My OS is on my primary HD too. My boot time was cut by a significant amount by using a 7,200. Also if you do any photo/video editing you MUST have at least a 7,200. Capturing video and reading the scratch disks depends heavily on HD speed.