The HAMMER would give MI real-time access to what the bad actors are doing. They can still use it because Trump apparently left it in place and hasn't given any signs that he knows anything about it. It provides all of their past actions and also their plans. It also shows who the bad actors are.  The discovery of the HAMMER is why Adm Rogers started a full compliance audit of the NSA in 2015. He had already been secretly logging NSA queries that stemmed from the HAMMER by this time. He informed Trump about it when he visited him at Trump Towers soon after the 2016 election. Logic suggests that Trump took this opportunity to use the system to turn the tables on the bad actors by mirroring the HAMMER and using it to monitor them.