Water Productivity: are we really sure about this?

Water productivity - the amount of a crop produced per unit water - is a much used measurement.
A quick search for the phrase on Google Scholar yields nearly 18,000 citations. So it is popular, but is it of any use? Former IWMI (International Water Management Institute) Deputy Director General, Dennis Wichelns, thinks it may not be. He believes that to distil crop production down to merely the amount of water used leaves a lot of important information out of the picture. Fertilizer, seeds, plant intakes, labor and unpredictable influences like the weather all play their part, but are ignored by such a simplistic ratio.
But if the calculation has such shortcomings, why is the mantra of "more crop per drop" so seductive?

[video via Think Beyond the Tap | IWMI]