hmmm.. i like the ideas about using class 0 planets for something, but i think it would have to come from a different tech string - not to mention they would have to create aquatic uninhabitable, radioactive etc.
This could add a lot of content to the game, and as people said theres loads of things they could be used for (heres some of mine, and a few that i liked that have been mentioned already)
-Refuelling stations, be it a space station orbiting the planet, or an actual self contained building on the planet, refuelling stations could add ship range without adding any influence, they could be 'captured' by enemies and a line of techs could make yours more secure!
-Mines, who says we can only mine asteroids? I'm sure that any planet will be just as usefull to mine, but then again there will be gravity issues and whatnot so space mining 2/3 could allow you to do that?
-Sensor hubs and relays, after all any civilistaion in space will want to keep an eye on its territories, so they would inevitably want to build sensor stations in outlying systems that are likely to avoid enemy sightings, and in the spy sense that others have mentioned before wouldnt it be cool to spy on New Iconia 3 from 4? decode enemy messages and generally spy!
-When you go to attack a system you look for enemy ships in space and orbiting a planet, but wouldn't it be a lot harder to look for a ship that was hiding on an uninhabitable planet? Of course such ships would require specialised landing gear to land on a planet with say, no atmosphere.
-Research stations, as aforementioned these could be really hany places to mess with some pretty dangerous techs!