ISS, HST, Spacelab research missions, numerous satellites are to name just a few. We understand far more about how to live & work in space now, than we did at the end of Apollo.
Skylab could have been used and was already used to understand how to live in space longer. The Space Shuttle development caused to abandon Skylab, although there was plans to continue using it together with the Shuttle. Apollo, which includes Skylab, was on a way already for living in space and learn how to go to to Mars.
The HST: really required a Shuttle because it initially was a piece of useless space junk. It could have been launched without a Shuttle, but it would have been nothing more than a joke. But telescopes and satallites actually do not require a Shuttle. The Shuttle was not even used to carry satallites back to earth.
Spacelab and ISS: a lab that is being in space for as along as the Shuttle does not make big sense, and the science is very little. It was just a Shuttle justifier. Look: we have done science, we have done somthing within the huge payload bay. Same for ISS, which is a giant waste of money. Remember that the Russians and Europeans did already live in the Mir space station for long periods, which was being assembled without any Shuttle initially. Anyway, the science aboard the ISS is very little and also just a justifier. In terms of engineering the ISS is a big project. But in terms of science it's really a waste of money. The ISS required more than a decade for being assembled, to do little science to show the public and politics the money is "used", but probably being deorbited not so long in the future.
The Shuttle era should have come to its end already in the midd/end 1990's and Constellation should have taken place by then. But NASA sadly missed it. That was the second mistake which lead to the current problems. Not Ares is in a mess, the Shuttle program caused NASA being in a mess. It causes a big gap once again, after the Shuttle all in all already has caused a decade of not going into space manned.
Space exploration is about more than simply going somewhere. It's about developing technologies to get there & live there.
All we do for decades, manned, is ionosphere exploration but not space exploration. It's nothing more than in case Columbus would have played with a model ship in a pool...
Nobody is questioning its design. As a rocket to launch humans into LEO, it is very good (ignoring the few problems it is currently experiencing, which could be fixed with expertise, time & money). What people are questioning is it's need. Why do we need it when, by the time it comes into service, there will only be 3 years max left in the ISS (if it hasn't already been de-orbited by then), and we will also by then have other methods of launching crews to the ISS anyway (ESA ATV, SpaceX Dragon, Orbital Cygnus).
ISS is not the wide fufure of manned space flight. It is just a big temporary waste of money.
What is the need of Soyuz, ESA ATV, SpaceX Dragon and others once the ISS is history? Right. That all is going to become useless, while Constellation is looking farther into the future of manned
space sxploration.
And as for its use in the Lunar program (if that even happens now), then, as Ares V would be needed anyway, would it not be much easier, simpler, quicker & cheaper just to put Orion atop Ares V, and that way you only have to pay for & support one launch, instead of two?
Two launches are required because AresV does not have the capability to launch Orion, Altair and the earth departure stage into LEO altogether. Launching Orion on top of Ares1 makes absolutely sense. Launching Orion on top of AresV would be disproportional, like using DIRECT just to keep expensive jops and infrastructure.
Ares1 is the most perfect Orion booster. NASA is on the best way since Apollo. And if that gets ruined now, be sure we'll be able to say: that's it. No NASA in space, well manned, for ages.