My current terraformer fleet consists of 19 ships (2x Genesis III, 8x Genesis II, 9x Genesis I), representing a total tonnage of 3,415,850 tons, with another four III's (another 3,374,800 tons) on the way, and I don't want to deal with the micromanagement of that many tugs. I've already mentioned before that I tend to prefer a fleet system that doesn't rely on heavily interconnected relations between various elements, both to reduce micromanagement, and to make each individual component more self-reliant. Not to mention that producing enough tugs to conduct a transfer of my terraforming fleet through the 12 connections to their next target in any sort of reasonable timespan would be a fairly large drain on my mineral income and shipbuilding capacity.
So basically, if I wanted to replace the engines and fuel with a single orbhab module on each of those, I would end up with slightly lower tonnage on the IIIs, and substantially higher tonnage on the IIs and Is, for roughly the same tonnage total, and would have to come up with enough tugs to move 6,790,650 tons of ship through dozens of billions of kilometers. Not worth the effort, at least IMO, when I can strap some engines and fuel tanks on and call it a day. Added bonus: they can also act as a refueling station for ships operating further up whatever warp chain they're working in.