In Systems of Survival, the late Jane Jacobs pointed out that when government takes control of commerce, the problems of Soviet Communism may arise: severe lack of competition, innovation, and incentive. When commerce takes control of government, the result may reflect the worst abuses of notorious “company towns”, where workers can become indistinguishable from slaves; it is also likely then that consumers will suffer from higher prices, unchecked deception, and lack of competition.
When Guardian and Commercial systems are mixed, you can get something like the Mafia: engaged in both commerce and force, willing to destroy in order to advance its goals, with no financial checks and balances on its financial activities and few legal checks on its forcible (criminal) activities. Another common result of mixing principles is a government where everything is for sale—you can literally get away with murder if you know whom to bribe.
This understanding provides a broad foundation for public policy. To solve problems related to the minimizing of harm (theft, invasion), an organization following the Guardian system is best. To solve problems related to maximizing wealth (trade, invention), an organization with Commercial principles is preferable. And an organization that mixes the two systems—for example, one that makes laws but is subject to financial influence—is likely to be dangerous, or at least counterproductive.
It's also worth noting that a large Commercial monopoly may take on Guardian characteristics—using deceptive or unfair business practices, threatening legal action to harm competitors, and buying favorable laws. If the government is willing to be bought, consumers and citizens will have no protection and unethical conditions will multiply. “Free market” must not be implemented to the extent that Commercial entities are allowed to engage in Guardian behavior.
One of the most important concepts in Jacobs' theory is that of “monstrous moral hybrids”, which are created when an organization tries to adopt inappropriate principles, or systems of action. A government that took bribes would certainly qualify, as would a government that tried to regulate all commerce by force and central planning. Both problems arise from a Guardian entity involving itself too much with money.
Even a subtle mixture can have distinctly bad effects. Jacobs gives the example of a police department that tried to make itself more efficient by giving bonuses to police officers for making arrests. Note that “Be efficient” is a Commercial and not a Guardian principle (see table), and Guardians are supposed to “Shun trading.” The result was predictable in hindsight: many false arrests were made in order to get the bonuses. The Mafia engages in trade and force both, using loyalty, greed, and coercion as motivators. Most people outside of the Mafia would agree that it is monstrous.