Taking sloanjh's suggestion and returning to Rich.h's original question, a good use for mineral-free habitable worlds is to serve as farm teams for factory worlds.
If you're like me, and you keep some significant portion of your homeworld's factory capacity busy building more factories, then you'll need more population to keep up with the demand for labor. In my games, once Luna, Mars, And the Galilean moons have populations over 25 million, I set up the immigration settings on the Civilian tab to Supply colonists from those worlds and Demand them back on Earth.
That's a quicker way to get to a large population than just letting them grow naturally, although I've never gotten to a trillion.