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.
I get what your saying but that would become an unfair advantage in competitive multiplayer if that is true. Hopefully, you have a choice to fight other players because if not many people will go around just wrecking newer players' ships. Which would make the game unattractive to people who don't own the product. I do not want it to become like LoL.
While it could cause an unfair advantage in a unranked competitive multiplayer i'm sure in a ranked/matchmade game there would be limits or requirements as to what can be used. From what i've read a large part of interactions with other players is what will make this enjoyable so i could not see them let the weaker player avoid combat, but by the same token many other systems could be implemented to make the stronger people not hunt the weaker people, one such thing being player bounty if someone engages ship combat without some form of request.