While I'm on board with metric, this technique is used in countries where the metric system is the only system too, because our intuition fails us at scales that we are not accustomed to, no matter the unit, and for other reasons.
On its face I'm not against using analogies to explain scale, but its not like i really have an intuitive grasp on the size of a 747 or a football field's volume. It also helps to remember that 1m^3 is 1000L which allows you to transform this into much easier (at least to me) analogies.
