You make a very good point, and my problem with McKinsey study was its bundling of various industries without subsequent breakdown by field. While it seems natural that customer-facing enterprises benefit from diversity (sales people, nurses, customer support), where a customer might seek out a familiar face, the analogy seems to stretch when you stretch it to other areas (tractor manufacturing, winemaking, etc.)

P.S. The McKinsey article is behind a wall, not paywall, just a registered account with mckinsey.com