Hardly a week goes by without some titillating new report devoted to tracking billionaires. There’s the Bloomberg Billionaires Index, the Billionaire Census, and others like the Hurun report out of China and RBC Wealth Management’s World Wealth Report. The lists are rising in prominence in tandem with the number of billionaires, which has doubled in eight years to 1,645, according to Forbes, the granddaddy of this cottage industry.

Beyond providing vicarious thrills, some of this billionaire data is drifting into serious economic...