Michael loses nothing by telling Fredo of Roth's plan to assassinate him. By that point both Roth and Michael knew each was a danger to the other. Michael also knew that in all likelihood Roth would not be joining him and all the big shots for Fredo's "night on the town." Moreover, it does not appear Fredo ever had the chance to tell Roth.

I am in the camp that Michael had Fredo on his suspect list pretty early on. The loaded question about Johnny Ola gives this away, and Fredo's reaction to Michael's invitation to go spend time together is to confess that "I was mad at you" and
his very anguished statement about the two of them not having spent time like this before. To me this i a dead giveaway that Fredo is the traitor, and it gives Michael the perfect chance to bring his enemies in closer by telling him about the asassination plot.