Yes the police can use them still. Their best strategy is to take the information from the CI and build their own case/evidence from it.  Sometimes they intentionally turn a blind eye to the CI's current crimes and then use it at a later time to invalidate whatever promises or consideration the CI was going to get.  This way the police get the best of both worlds: They get the information and they get to tell the CI that their deal is off because of new crimes.