-- Con --
(ie: pro-One Matrix)
In the movie, Agent Smith says: "Did you know that the First Matrix was designed to be a perfect Human world where none suffered, where everyone would be happy? It was a disaster. No one would accept the program. Entire crops were lost."
The machines recreated the Matrix and modeled it after the peak of human civilization. One would think that they would try to maintain as many of the complexities of the real world to keep the virtual prisoners distracted from their true condition.
Also, the problem of computing power (speed) can be reduced from a purely quadratic growth function to a more linear one. Suppose that there is a limit to human perception. One can't percieve the breath of the person sleeping in another room. (Unless, perhaps, they are a loud snorer.)
The Matrix could draw "sandboxes" around groups of people sharing an experience. People isolated enough from eachother wouldn't have to share immediate experiences -- only when one entered into the other's "sanndbox". In this way, the Matrix can create many "sandboxes" and use a type of SMP to process separate groups' experiences.