"Please don't pick the most provocative thing in an article or post to complain about in the thread. Find something interesting to respond to instead."
I don't think the metaphors would be evaded by using metric units, though. Would 4 Teraliters been more useful to much of anyone?
Converting it to gallons, liters, or any other common unit of volume probably leaves most people without a sense of relevant scale unless you use a metaphor (e.g. how many households it could sustain for a year) or otherwise tell them how much water the states use (e.g. including a percentage of total usage).
It's a point of reference. I'd argue it's unnecessary, but people still use points of reference in the metric system. Sure, a hectare is simply 100m by 100m, but people will still use their personal memory of a space they know when thinking about a hectare or a kilometer, or whatever. People understand scale relative to something familiar.
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.
