Interesting discussion, but I still scratch my head about what makes deep-space commerce "profitable" or desirable. I can understand if some uber-valuable resource is only found on certain worlds/asteroids, like dilithium. But just general ores, and other substances? Doesn't make sense to me.
We know that dilithium is needed for power and starship propulsion. We know it is very rare. This, therefore, makes it extremely valuable. So wherever you find it, it is time to mine it and sell it.
But such a valuable commodity could also pose a security risk. Remember this exchange between miner Childress and Captain Kirk in "Mudd's Women":
CHILDRESS: You've got no choice, Kirk. You beam a landing party down, and you won't find one blessed crystal.
KIRK: No deal. You're a long way out in space, gentlemen. You'll need medical help, cargo runs, starship protection. You want to consider those facts too?
So it is logical to consider that even profitable mining and/or manufacturing colonies would need help and protection in the STAR TREK Universe.
The thing that gets me is that there has to be a sustainable infrastructure of space vessels (not just robot cargo ships, but also "starship protection") in order to make trade worthwhile and secure. That means not only do you have to pay for expeditions (the building of ships, trained astronauts, trained colony specialists, equipment, supplies, maintenance/repairs, etc.) but you also have to pay for "starship protection" even once the colony has established itself. All of this sounds very, very expensive to build and maintain.
I'm not saying it never makes sense in TREK; just that it's not the only way.
What if some colonies start out as penal colonies or planetary starbases, or even as research colonies? If these kinds of settlements evolve into "worlds", where people choose to settle because they want to live on "strange new worlds" that are unspoiled, that would tie in with what Picard was saying in "Justice".