I believe that he uses one in order to be a Green Lantern, he doesn't or didn't need it at the time, because during the Blackest Night he could use the ring, AND use his blue aura. Now however, I'm not so sure. It is possible that he has biologically locked himself from his natural powers. Guardians are capable of augmenting how powerful they are because they can manipulate their own biology at will.
DanielGleebits's forum posts
Well given that the Guardians are near omnipotent, have defeated and imprisoned entities such as the embodiment of fear, imprisoned the majority of all magic in the universe, and defeated the Anti-Monitor when he was at his first level of strength, I think the Guardians could defeat him. It's not really a test of overall strength, it's the fact that the Guardian's powers are attuned to defeat beings over supernatural power.
Of course, people will say things like "well Scar beat all of them single handed". Yes, she did, but given that she had the same level of power as them, and then the added power of Nekron, who is perhaps the most powerful entity in the entire DC Universe, I think that's kind of a power distance there. Also, Scar was only defeated because of her natural weakness to the powers of the Emotional Electromagnetic Spectrum, otherwise she would not have been destroyed.
Given that the Guardians can take direct assaults from Superboy Prime with little to no damage, I don' t think it's fair to count them out just out of hand