None of the criticisms specifically challenges the need to design your network pro-actively. This criticism, however, skirts this concept very closely. In real life, though, the 10 movie-goers will arrive at irregular intervals. Perhaps during the first 10 seconds, three customers show up, then during the next 40 seconds no one arrives, and during the next 10 seconds another four customers arrive, making an average of six customers per minute. These and other names have been the frequent source of headaches to entry-level simulation practitioners.
Formulas exist to solve the average service, and wait times for scenarios like the one discussed.
Furthermore, formulas only give answers in cases where a steady-state has been reached. In real life cases, simulation is often needed and cannot be substituted by simple napkin arithmetic . A second aspect is that actual system measurements do not always report congestion situations. When you measure the utilization of a Wide Area Network link, you simply do not know if some packets have been dropped down the line because a router was congested. If you measure the router utilization, you just don't know if an improperly configured entry is affecting the measured performance.