If they weren't under constant thrust, how long would you expect them to accelerate for before they stopped thrusting?
As long as was most efficient. The problem with rockets is that there's a point of diminishing returns -- the more fuel you carry to thrust with, the heavier your ship is, so the more thrust you need, so the more fuel you need, so the heavier that makes your ship, etc. So anything that reduces the amount of thrusting you need to do is a good thing. The advantage of being in space is that you don't need constant thrust to stay in motion, so it's just good sense to take advantage of that. I prefer fiction that embraces the, well, spaciness of space, the novelty of its conditions and how people would function in them, rather than trying to transplant our familiar ground-based experiences and assumptions out there. Sure, you could argue that humans need to be under fairly steady gravity for our health, but it's more efficient to achieve that with a rotating habitat section (something I'm surprised the
Expanse ships don't have) than relying exclusively on rocket thrust.
In my novel
Only Superhuman, set in a Solar system setting not unlike the
Expanse books, ships tend to rely on a variety of different propulsion systems, many of which don't require the ship itself to thrust at all -- e.g. momentum-transfer tether stations ("Bolasats") and magnetic sails impelled by particle beams ("drive beams") fired from the various space habitats (doubling as braking sails using the magnetic field of a planet or the Sun). Both of these can easily impart thrust of several gees for brief periods, if needed, and together they constitute an established systemwide transit infrastructure serving as a sort of "interstate highway" system to facilitate travel. There are high-efficiency plasma drives for when you're really in a hurry and nothing else will do, but the fastest and most capable ships are the ones that can take advantage of as many different propulsion methods as possible -- e.g. getting a push Sunward from a drive beam, letting Solar gravity accelerate and slingshot you, thrusting hard on plasma drive to maintain momentum as you rise out of the Sun's gravity well, then catching a Bolasat at a fairly high tether radius so it'll swing you on course for your target habitat, then braking against its drive beam to slow for rendezvous. So shipboard "gravity" tends to be highly variable and intermittent.
