Most of these kinds of simulations aren't expert systems but microsimulations (cf. http://en.wikipedia.org/wiki/Microsimulation http://en.wikipedia.org/wiki/Mic

Most of these kinds of simulations aren't expert systems but microsimulations (cf. http://en.wikipedia.org/wiki/Microsimulation http://en.wikipedia.org/wiki/Microsimulation) and they are actually used quite a lot for certain kinds of problems, like figuring out the indirect effects of changes in taxation. It is often possible to check baseline accuracy (does it at least work in current conditions?) but as for predictive accuracy, well...