When the Shuttle was designed, one of the missions intended for it was to service military satellites, or more accurately, NRO satellites. That's why they spent billions setting up SLC-6 at Vandenberg AFB, CA. Military and intelligence related satellites in the low Earth orbits within the range of the Shuttle are almost all in sun synchronous orbits and you can't reach those from the Cape. Following the Challenger explosion, plans to launch Shuttles out of SLC-6 were scrapped due to safety concerns like hydrogen buildup. In the end, other than providing a few launches, the Shuttle never did service any military related satellites.

A couple years ago, I attended a presentation on what the Air Force and NASA wanted in the way of a new space plane. The goal was to identify common needs and technologies to make such a project happen. The Air Force wasn't convinced it needed a space plane at all, but if they did, they wanted something unmanned that could be launched with hours or days. It would carry a payload of perhaps 5,000 to 10,000 pounds to LEO and then quickly reenter. On the other hand, NASA wanted Shuttle II, capable of lifting 40,000+ pounds to LEO. They didn't care if it took weeks or even months to prepare for flight. And of course, it would be manned. The final result was that there wasn't a lot of common ground.

When it comes to launching payloads, there really isn't much need to send up humans. Expendible boosters do that job just fine and for a lot less money. To carry crews between the ground and LEO, you really only need a vehicle capable of carrying a few (4-6) people and perhaps a few thousand pounds of logistics supplies. For medium/large payloads, there are already boosters like the Proton, Zenit, Ariane V, Atlas V Heavy, and Delta IV Heavy that are already operational or will soon be available. For really big payloads (100,000+ pounds), there can be derivatives of Shuttle technology or even a revival of the Energyia for that mission.