@hashem: The reason for this assumption is to focus on different aspects of the problem. The market secures the greatest happiness for the greatest number in the process of operating. It is true that the market is not a person and does not consciously aim at this end. But a socialist central planner is a person. Let us assume, therefore, in order to give the socialist the strongest possible argument, that Smith the planner also is intensely interested in the good of the whole society.
Now as a matter of fact, the planner is not so interested. The assumption is false. Here’s Mises: “The real Fuhrer, however, turns out to be a mortal man who first of all aims at the perpetuation of his own supremacy and that of his kin, his friends, and his party. As far as he may resort to unpopular measures, he does so for the sake of these objectives. He does not invest and accumulate capital. He constructs fortresses and equips armies.”
But the case against socialism does not end here but spans several issues. We assume the planner’s omnibenevolence in order to zero in on the other problems of socialism.
That socialism is sequential, while capitalism is parallel, is as much a distinction as that socialism is disinterested in the common good, while the market is interested in it. In other words, under capitalism, people’s search for private profits redounds to the common good. Under socialism, Smith the planner’s pursuit of his own happiness does not do that. Again, this is a very important point. But it is not the only one.