while one can postulate that this level of civilization doesn't need planets as habitats, planets with biomass would likely be an important asset as they ease the supply logistics and act as a trampoline for further, faster expansion.
But once your civilization isn’t threatened with complete and sudden obliteration by planet killers, you’re no longer stuck in the prisoner’s dilemma and you’re not necessarily compelled to strike first. It’s a big difference.
You shouldn't assume that an intelligent civilization wouldn't find a weapon of mass destruction that's effective against a non-planet based civilization.
So the utility cost of the betrayal scenario goes from negative infinity to a question mark.
Outer solar systems bodies have water and other volatiles as major constituents. If you want raw material for human bodies, cattle, trees, plastics, that is where most of it is.
miller experiment shows that there's some convergence on certain molecules, it's safe to assume that while every specific biology will differ, the building blocks will look not that far apart, and transforming existing biomass into usable biomass is going to be more energy efficient than transforming raws into usable molecules.
And so would interstellar bodies. I’m willing to bet that a large fraction of the mass of a developing solar system gets ejected during the early years of its formation. If you can freely roam among the stars you would probably find everything you need in interstellar space.