AMUSING, INTERESTING, OUTRAGEOUS, or PROFOUND

It all depends on what you're trying to do. Like, say we're talking about cars. With competing private companies, you can have one company that specializes in making the safest models, another that makes the fastest, and a third that makes the most fuel efficient. Different people have different preferences, so having more options makes sense, and cost-benefit analysis is a useful tool in looking at what features to cut or what research to invest in (provided you have enough regulation to prevent them from cutting out safety and turning the roads into Mad Max).

But suppose what you're trying to do is build a bridge. Well, the bridge pretty much just needs to do one thing, stay up. You don't need three different bridge companies building three different bridges in the same place trying to coner different demographics. You don't need the bridge to make a profit through tolls in order to pay for itself. You just need it to stay up. In cases like this, central/state planning makes a lot more sense.

Certain industries are "natural monopolies," meaning that redundancy is either impossible or extremely inefficient. An example of this is cable. It usually doesn't make sense for two different cable companies build their own whole infrastructure in the same place, and so most places end up stuck with a single choice. In that case, it's better for the industry to be state run, because then there is at least some mechanism for consumer feedback through the government, compared to a corporation with a monopoly that will just do whatever it wants.

There are reasons why large corporations have become such a large part of the economy, and part of it is that it's often more efficient to coordinate things on a larger scale. Sears is a famous example that tried to do things differently. They hired a libertarian CEO who hated the centralized organization of the company and restructured it to have individual chains and departments competing with each other instead of coordinating. It was an unmitigated disaster. It's just a matter of economics of scale.

Personally, I think a mixed economy makes a lot of sense so that the best models can be applied on a case-by-case basis, provided that corporations can be kept in line and prevented from doing regulatory capture. That's why I'm a fan of China's model. "It doesn't matter if the cat is black or white so long as it catches mice."