It's important to remember that although Time Machine is touted as a great way to back up and recover your individual files, it is also a real backup solution which allows you to recover an entire hard drive to a particular point in time.
Duncan's test provides an excellent example of what a user can do if their main drive goes dead and they need to recover.
This technique is also very useful for preventing problems with OS upgrades. I wouldn't suggest this as a way of going back and forth for most people, but you should always make sure your Time Machine is up to date before you perform an OS upgrade. If something catastrophic happens, you can always use the boot disk to get back to your previous configuration.