Put another way, why would neohumans run detailed world simulations?
Not being neohumans ourselves, we can only speculate. However, it is interesting to extrapolate from the reasons we run (relatively low-detail) simulations today. Flight and automobile simulators have been available both to the military and for entertainment for many years. Financial simulators have become important for investors, as medical simulators have improved our ability to train surgeons. Many persons enjoy playing games such as SimCity that simulate urban planning. Entire worlds are simulated, for both entertainment and scientific purposes. In the popular virtual world Second Life, persons buy and sell real estate, hold meetings, even dance, and generally engage in a virtual life through the proxy of their avatars. Although also intended for entertainment, World of Warcraft may serve to rehearse real-world epidemics. Another virtual world, designed by Simulex for the United States military, is now modeling Earth down to the detail of individual persons.
Ultimately, however, I wonder whether neohumans' relation to their simulations might be better reflected in the relation between human parents and children. As we have progressed technologically, our relationship with our technology has become increasingly intimate. Computers that were once far away in warehouses are now in our pockets, or even embedded in our flesh. Whereas we originally used them to crunch numbers, we now use them to bring us together in ways never before possible. I anticipate that this trend will continue, and fully expect that we will one day become indistinguishable from our technology. Our future selves will relate with our technology not as we now relate to the boxed computers on our desks, but more like the simulations that are constantly running in our current biological brains. For such sublime beings, whose thoughts are more detailed than the most powerful of contemporary simulations, a procreative act sufficient for reproducing another of their kind may entail nothing less than the synthesis of new worlds, calculated perhaps according to some sort of cosmic heredity (increasing the likelihood of fine-tuned physical laws) in preparation for a Darwinian dance of eternal magnitude.