Besides the added complexity of assembling components together in LEO, placing any mass into low Earth orbit is really inconsistent with putting it into a trans Mars trajectory and adds a giant extra delta-V penalty. It's a much different orbital inclination, plus there's the added energy loss of circularizing a trajectory into low-Earth orbit and later boosting out into a hyperbolic escape trajectory. As far as I understand it, it would be faster and cheaper to develop a true heavy lift launcher.

Why do you say the inclination is different? You can go into LEO at any inclination.

I also don't see why you need more delta-v to go from launch orbit to circular orbit to transfer orbit compared to the transfer orbit directly. My orbital mechanics are rusty, but it seems to me that since you need to lift the perigee from Earth's surface to Mars orbit, you'll pass the circular orbit on the way.

If you go into low Earth orbit from the Earth's surface, you need to go into an orbital inclination equal to the latitude of your launch site - which is extremely unhelpful if you want to then take that mass anywhere else in the solar system. You also need to circularize your orbit, then later boost that mass from circular orbit to hyperbolic escape orbit - also extremely wasteful.

Ok, good point about the inclination.

I still don't see why circularizing the orbit would be wasteful. Circularizing an elliptical orbit at apogee still requires positive delta-v, which will help you on the way to the hyperbolic orbit. Only if you want to circularize at perigee would you need to slow down, which would indeed be extremely wasteful. It's true that you get more energy boost for the delta-v if you do it early rather than late, but I don't know if the ellipticity of the launch orbit is high enough for that difference to qualify as "extremely" wasteful. But maybe it is.

True, a better Saturn V would be the best. I was just trying to think about, if for whatever reasons, a better rocket would not get ready in time. But maybe what I wrote was just plain bullshit. ;-) Of course it would make more sense to send all the small rockets towards mars at the same time and 'somehow' combine them but that doesn't sound easy either.

Why bother putting it into a circular orbit? I don't see the point.

As I understand it the big drawback to EOR from an engineering standpoint is that the added complexity introduces a bunch of extra failure modes.

If you boost something up to orbital velocity and LEO altitude but don't circularize, its orbit still intersects the atmosphere, if not the earth. The delta-v required to keep from falling back down would be better used to push into whatever transfer orbit you want.

Perhaps Don't assemble them - launch them separately and they arrive separately.

To some extend thats of course possible. Wouldn't that be funny if in the end, all crew member travel separately because its cheaper? :-D Now thats isolation! At least they would have short latency when talking to each other.

Of course, once you've build a big enough rocket, its usually cheaper than multiple small ones. But there is a market for medium size rockets like the Falcon Heavy already. Not so, for bigger rockets - yet.