I know this is never going to happen, but I really wish that when they do these secret powers they put a caveat on them that they are not allowed to tell anyone or else they lose it. Production wants them to tell people and probably even push them to in the DR. Now there are some that will argue that if production wanted everyone to know, they would just not make it secret. These people are idiots. It creates more drama when it is supposed to be a secret and that is what they're looking for.
Now it's easy for us to say we wouldn't tell anyone as we are not in the house and dealing with everything else they have to deal with the pressure they're under. I give MJ credit for not using her veto on Matt, especially this early in the game. Both Quinn and MJ should have the same plan for their respective powers. They are both good through the first four weeks. If it's me, I'm keeping htat thing in my pocket the entire time and would only use it on someone other than myself if it is the 4th week and about to expire. Yes, it could be looked at as selfish and not being a team player for the alliance (which is why you keep your mouth shut and not tell anyone), but you need to be selfish to win this game.
Hell, lets say MJ keeps it to herself and ends up using it on Matt in week 4 as it's about to expire. I'd still keep the whole thing to myself and let everyone wonder who it was that saved him. The only thing to gain by letting everyone know is that you kept a secret from the rest of the alliance so what other secrets are you keeping?