If the US wants to go to Mars for more than a single short mission, it's going to need the ISS or a replacement. We'll need to be able to build ships in orbit so they aren't limited by the constraints of the first hundred or so miles of the trip (lifting the ship up from the surface to Earth orbit), that's the only way we'll be able to build them large enough for the crew, supplies and equipment needed for a mission of more than a week or two. And if we want this to be a sustained thing, sending more than a couple-three missions, we're going to need to be able to build ships without shipping the majority of their components up from surface.
And the ISS will help how, exactly? The entire ISS came from the Earth's surface. Unless you have a really fancy plan to do asteroid/lunar mining, that's where all future materials will ultimately come from too. The ISS is way, way down in Earth's gravity well so if you could do mining you wouldn't build it there anyway. We can assemble a ship in orbit with or without the ISS, nothing really gets easier. What we're building must have a crew module, so any astronauts working on assembly can just live there. Not that I really see the need, the assemblies could dock like spaceships do and just interlock with bolts.
Star Trek has ruined a generation's sanity when it comes to space stations. The only reason you'd want a space station is so you can have a ship come in for maintenance, repair, upgrades or refueling in orbit so they don't have to go down the gravity well. If all you're doing is sending ships out never to return, it's a total waste of time. Unless you get to the point where you have a shuttle taking things from Earth orbit and Mars orbit and returning for a refuel it doesn't make sense. And it probably doesn't make sense unless you can refuel in Mars orbit. Which means it's not happening in this century.
Space stations are not like gas stations where you just drop by as you pass one by. Unless you're planning to be in Earth orbit, entering Earth orbit to dock with the ISS and deorbiting to get to your destination costs a helluva lot of fuel. And that is the crux of the issue, it almost never makes sense to build a waypoint into your route as opposed to just going to whereever you were planning to go in the first place. If possible you might not even want to assemble in oribt, just launch multiple rockets on the same trajectory and have the bits assemble in zero g before firing off to their final destination.