5000km/s*18 hours = 324 million km, gets you a little bit further then "Earth-moon runs only", doesn't it?
5000 km/s is quite fast for the sort of early-game situations where you're going to be looking at short-range colonization. In the sort of case where you're seriously colonizing Mars (as opposed to doing it for free money), it's going to be days away.
It would also be much cheaper and give you much greater capacity over shorter colony runs + require no extra tech research. ( which is the main reasons to implement something like this ).
I will agree that it might be nice for Conventional starts to have something that's between the Luxury Passenger module and the Cryo module.
For example in the current fiction in Colonial Wars just mass evacuating everyone from Earth to for example Mars could be an option to consider if this kind of modules were available.
Infrastructure would bottleneck that very quickly. Actually, you'd be better off shipping 25 million to Mars, then taking the infrastructure the lines haul there back to Earth.
Lack of capacity/loading times isn't really a realistic argument against this either IMO seeing how we have airports with our non TN tech today that handles 50-100 million PAX per year = an entire sizable Aurora colony.
A big airliner carries ~400 people. An Aurora cryo module starts at 10,000. I can't see a ship with less than, say, 40,000 being efficient. So that's 100 times the size of an airliner, which takes at least half an hour to load and unload. TN tech isn't really going to help without adding lots of complexity. Sure, we could divide up into ~400 passenger sections, but this is where the square-cube law bites you. I can't see getting good access to more than about 4 wide-body equivalents on a section, maybe 6 at the outside. Past that, you're going to start interfering with each other. So either it has to be 18 times as long as an airliner and only 3 times as wide (which is absurdly skinny), or you can't unload at airliner speeds, or you can't keep it as simple as you want it to be.