Pretty sure you ca use ships to fight because I read in a Joystiq article that one of the Bungie representatives said interesting encounters with others will occur upon recieving the question if space battles will be an in-game feature. Other evidence for space combat I got from the same article. The reporter wrote he was told of a situation where someone met their friend at "The Tower" and proceeded to"The HANGAR" so that they could embark on their quest. The reporter goes on to elaborate that the person noticed their friend had been doing well in competitive because he had bought a new ship. Since you are able to buy new ships i'm pretty sure you can fight people with them.