Remember, the original Rockwall design concepts for the shuttle were SSTO. Reason you ended up with two strap ons was because of a parallel soviet effort we wanted to beat. So instead of a reusable design we ended up with a kludge. One could argue we got in-flight satellite servicing - which is a capability we can't duplicate without a version 2 orbiter. But the throw weight of the orbiter was way below the original specification and what we ended up with was more out of expediency then a rational design criteria.