Central Planning

Let's begin with the central planner's view. Since radicals don't express themselves in these terms, I will rely on a neoclassical economist's translation of what he believed a prominent Marxist would have said (about 25 years ago) if he had used the terminology. I won't quote directly, but it went something like this:

Public goods, quasipublic goods, and externalities are the real world of the market system.
These 'problems' are so pervasive that the only hope for an efficient allocation of resources is for the government to take control of the allocation of resources, do the statistical work necessary to discover the social costs and benefits, solve for the optimal allocation of resources, and direct the managers of the economy to realize it.
It's true that this is very difficult -- the Soviet Union clearly never came anywhere close. But even if the plan is pretty far off from the optimum, it can't be much worse than a market economy riddled with inefficiencies due to externalities and underprovided public goods.

The deterioration of the Soviet economy in its last years makes this last point pretty difficult to swallow, of course, but this is one possible interpretation of public goods and externalities.