It's what we have. My concern is this habit of starting something--then someone killing it--then starting something else, and its enemies killing that. SLS won't have that pesky orbiter to contend with. The SLS up mass will be all payload. This means that ISS would actually have been done more quickly with fewer large modules than with a lot of smaller 20 ton modules. That puts it ahead of an STS that wasted a lot of power on placing an orbiter in LEO.
I just don't like the idea of a lot of liquid sloshing around up there in a depot with boil-off problems. Now newtype wants a return to hypergolics--and a hypergolic depot would be fine--but we aren't going down that path. That decision has been made. The plus that comes from larger LVs for exploration is that you do all your fueling on ground level--put a heck of an upper stage up there with a big rocket--and get rid of those liquids as fast as you can. Things do wear in space--we saw that with LDEF and ISS solar panels, micrometeoroids, etc.
The plan is political, but there is nothing to say that if STS can fly 7 times of year with an orbiter--that SLS couldn't match that without one. That's a budgetary question--but that affects everything. We might get a president that destoys NASA--and then we get only what Musk can fund privately--and his LV was 80-90% gov't, so is that really private spaceflight if he is just another contactor?
Having 70 tons in space (orbiter free) allows for simpler missions, no Rube Goldberg assembly --at least for awhile--until we can afford a Mars ship that is more responsible than Musk's laughable one-way missions. I've been a big fan of Cassini's Ms. Porco, and she and others see easier sample return missions, icy moon landers and the like coming from SLS. It is capability--not frequency of launch--that is most important.
STS was an HLLV with only a Titan IV payload--and thus redundant. Free it of the orbiter, and you open up BEO in a responsible and engineering friendly fashion without depots that could easily make space debris worse by being a prolonged target to expansion, debris hits, etc. I don't want one of those things blowing up and making LEO a minefield.
Let's put that off--or at least have the depot a more sturdy SLS launched design with some meteor bumpers, and not an EELV launched balloon tank eggshell of a Centaur. That spooks the crap out of me. We had a small Briz failure a few years ago and it put more debris up there than the China ASAT test and the US sat-shootdown combined.
We have a space race exactly because R-7 was more than an ICBM needed to be. Where we shrank the payload, the Soviet response was--make the rocket bigger.
And you know something? It worked.