@ Hans Neeson-Bumpsadese If there is a specialist subcontractor who can do something better than the prime, then I'd like to see that aspect of the work subcontracted out.
The above prompts the question "why didn't the subcontractor submit a bid to be the primary contractor then?"
While I don't actually disagree with you your example (of Ford v Cosworth engines) doesn't actually match the case you argued. Ford make engines, but contracted some specialist engine manufacturing to Cosworth. In that case Ford was the Principal, and Cosworth a Contractor. Ford was not a Contractor who subcontracted work to Cosworth.