My last game had a really kewl twist. ALL of the non-inhabitable planets within 2 SECTORS of one system became size 14 inhabitable planets!!!! It was rather late in the game, so I managed to colonize all of them, but none were able to develop because I was too close to winning. Think what would have happened had it been early on in the game!
So the non-inhabitables are (1) eye candy, (2) voted on by the UP to be used when they become habitable, (3) taken over by minor race, (4) taken over by a Dark Lord, and (5) able to become fantastic planets due to some rare quirk of radiation.