Terra said that he doesn't care about being Master, and that he'll use that power (darkness) to protect his friends. Remember that it was the darkness that prevented him from becoming Master in the first place.
Eh, in response to the actual question, Sora doesn't really need a Master, but there are certain things that he can't do because no one's ever been there to teach him. I guess I'll say Aqua.
Technically they had a Master who believed in absolute LIGHT so Eraqus was biased in his decision.