Maintaining very small ecosystems

Fair enough. However, for planet-based habitats, it would seem to be a
goal to strive for to terraform the entire planet rather than restricting
Mankind's ability to survive on a planet to biodomes and spacesuits. I am
sure that in the distant future when we have both free-floating colonies
as well as planet-based colonies (ah, to dream!), there will be reasons to
choose either option. It is hard to predict where technology will bring
us in the next few centuries, but given current technology trends, a
terraformed planet would seem preferable from a simple safety perspective
than an artificial body, even if it requires much more work to escape a
planet's gravity well.