A simultaneous optimization of the shape and trajectory will result in an optimal shape for the over all mission. The chance to fulfill the overall objective also after various design loops and redefinition of subsystems is much more increased. This is supported by sensitivity analyses of the design criteria. According this information design margins can be fine tuned.
Existing vehicle and mission studies can be be easily analyzed regarding extended mission objectives. Again sensitivity analyses will provide information about necessary system modifications. Various subsystem designs, like different propellant types, can be easily benchmarked as necessary modifications of the tanks, engines and shape can be considered.