At the moment, any planet that is uncolonizable is just dead space weight. They provide nothing to your empire, and nothing to the overall picture of the game. However, any true spacefaring civilization would have a lot of industry centered on these planets in order to harvest the resources, as well as the possibility of terraforming these planets into habitable areas. This is one area that I feel GC3 is very deficient on, and could be improved greatly.
To make a quick comparison, Distant Worlds has excellent game mechanics when it comes to developing uninhabitable worlds, and is in fact a central game mechanic. Now to be clear Im not asking for a private economy, resource mining stations etc (ala DW), but still I think something should be done with uninhabitable planets, because as it stands there is no reason to ever look/click/interact with these planets at all throughtout the game, and they make up a great % of the overall map.