If you will need more delta-V than you have on-board tankage for, sending another ship at the same time and acceleration, and refueling from it on the way, is overwhelmingly better than launching literally dozens of ships to get even more fuel parked at Mars, and then spending extra fuel stopping there to pick it up, and more again to get moving again.
If your true goal is to have crap on and in orbit around Mars, do that without pretending it has any other value. You don't fool anybody, but you make people wonder about you.
Or, hear me out, you get the available infrastructure set up on the surface of Mars to manufacture fuel and propellant, and then you can send a single ship from Mars to Jupiter. And, if you can bring payload, you can set up the infrastructure to do ISRU from one of the moons of Jupiter, and eventually bring your ships back with samples.
Versus, cannibalising several extremely expensive ships in order to get one ship to a destination it's never coming back from.
And this is just one example. The lower delta-v from Mars would allow extra payload to the belt, so if anyone ever intend to exploit those resources, starting from Mars will allow them to get heavy equipment there in less time and fewer trips (provided, of course, that they can manufacture that equipment on Mars).
You start by shipping a million tons of freight to Mars, while I do meaningful stuff at places that are not a waste of time and effort. By the time you are ready to supply fuel FOB at Mars, I have sent a million tons to places worth sending it to, instead.
Maybe my million tons of freight includes a liquid methane sump on Titan, where I don't even need to synthesize, never mind liquify the stuff; it is sloshing around in puddles everywhere. There is an interior liquid water ocean, and ice lava flows.
Seriously, if you need more delta-V for a Jupiter or Saturn trip than spaceship design A gives you, you are much better off making a spaceship design B with enough tankage to make the trip. (Maybe B is just A with an extra tank strapped on; or, a tug boosts A to escape velocity and then loops around Luna and back, aerobraking to LEO.) Park fuel depots at both low and high Earth orbits; you waste nothing by using those, unlike anything parked foolishly at Luna or Mars.
Luna, anyway, has stuff that is worth visiting, like craters in permanent shade at the south pole, and lava tubes where vapors have maybe drifted in and froze for hundreds of millions of years. Mars is the armpit of the Solar System.
You seem pretty contentious here; I'm just making a case that Mars makes sense as a destination, not trying to convince you that this is something you need to pay for. IMO, the numbers work out for it being a hub for traffic to the belt and outer planets. Maybe you don't agree. That's fine. Again, I'm not asking you, personally, to start backing this. Just stop spouting off nonsense like 'if you can't get somewhere from Earth, you can't get there from Mars'. It's a matter of fact that delta v requirements from Mars are lower than they are from Earth, and this would have been obvious if you had thought just for a second about any of this.
> You start by shipping a million tons of freight to Mars
The equipment for generating the electricity to perform ISRU on Mars is supposedly within the payload capabilities of a single Starship, 100-150 tons, not a million, and this should be enough for a ton per day, enough to resupply in the 26 month launch window. The equipment for collecting water and CO2 is another matter, and I don't see estimates on that, but with the above and some engineering margin, it seems plausible that this operation can get off the ground in under ten ship-trips to Mars.
I have said nothing even vaguely like that, and cannot guess where you got it from. But it is a simple fact that:
Any trip to the outer Solar System that stops at Mars is, energetically, much more costly that one which does not stop at Mars.
In addition, getting stuff to Mars is itself a huge expense that completely swamps any imagined benefit of extracting fuel and launching it to stop by for.
If you want to go to Mars, go without promoting obvious falsehoods about any value it has as a transport hub. It has none. Period.
If, to make sense, your Mars story needs for Mars to be a useful transport hub, then it fails, and you need a different story.
> If you can't get there without a stop at Mars, then you really, really can't get there with a stop at Mars. Stopping at Mars costs extra.
This is as asinine as saying, if you can't get somewhere without a stop at the gas station, then you really, really can't get there with a stop at the gas station. Stopping at the gas station costs extra.
After all, it costs fuel to take an exit on the freeway and go to the gas station. It costs fuel to leave and go back on the freeway. It costs extra! Energetically, any trip where you have to make a detour to a gas station, costs more than a trip where you don't make a stop there.
If your true goal is to have crap on and in orbit around Mars, do that without pretending it has any other value. You don't fool anybody, but you make people wonder about you.