Imagine if the shuttle were payload instead of the actual launcher. If the main engines were located on the bottom of the ET (Ares V), then the actual orbiter would be free to be improved over time. By having an independent heavy lift launcher, different generations of orbiter could be designed and built as technologies improved and as money permitted.

The heavy lift launcher could also be used for launching unmanned payloads and deep space probes in addition to orbiters. This would increase the rate of launches and drive down the cost per launch of the entire system, regardless of configuration.