Remember what the Dark One told Rumple, ” I know how to recognize a desperate soul.” Then Rumple told Belle “My power means more to me than you do dearie.”. I agree that he is protecting others from the “Curse” that the dagger possesses, an eternity of loneliness and doing the bidding of others even while imprissoned. I can see many hurt people in StoryBrooke, two with pained souls neither one of them deserving the “curse” of the dagger, David and Mary Margret.
If Ragina knew about it she would of had no reason to enact the curse, just become the new Dark One and make Snow Whites life a living hell that way. It seems to me that she and Rumple were friends in Fairytale land and he wants to protect the ones he cares about so I think he would have kept the source if his power hidden.