I subscribe to the Gossamer Gear newsletter ( which I like immensely) and they recently posted an article were two Scoutmasters used a 5 gallon bucket as a tool for the young Scouts and their parents to use as a reference for reducing the amount of gear they are planning to take on a hike. (http://gossamergear.com/wp/outdoor-education/first-steps-introductory-backpacking-with-scouts-and-the-5-gallon-challenge).
I think this is a great technique for many to use when starting out: cheap, reusable/sustainable, low tech, etc. Volume doesn't tell all, but it puts the brakes on over packing pretty quick.
The article noted that most were able to meet the mark if sleeping bags were left out. 5 gallons is just a bit under 19 liters, so that sounds reasonable to me.
It's not the end-all, but another arrow in the quiver for reducing pack weight. I could see preparing to take a newbie out hiking and handing them a bucket and telling them that all their clothes and toys need to fit, with some tutoring on layering, no cotton, personal hygiene, essentials, etc. Add hydration, sleeping gear, shelter, kitchen and food and you have a hike :)
