Suppose Alice and Bob are talking about a big drug deal over the phone, and Carol (working for the NSA) listens in. Carol lets her friend Dan know, and Dan's a police officer so he goes on a stakeout and catch Alice and Bob in the act, with $1M of cocaine in the trunk of Alice's car, and $1M of cash in the trunk of Bob's car. Open and shut case, right?
US case law states that the cocaine is "fruit of the poisonous tree", because the cocaine would never have been discovered by the police if it weren't for the illegal wiretap. Therefore it is inadmissible as evidence. The prosecution's case is sunk, and Alice and Bob go free. (Note that Dan never committed any crimes, might not be aware of the wiretap, and got good, hard, physical evidence that Alice and Bob were breaking the law.)
So if the government tries to use an illegal wiretap in an actual criminal case, it will probably do the prosecution more harm than good. Best chance for a conviction is to pretend the wiretaps never happened.
Edit: I think a good example here is "Nardone et al. v. United States." In this case, Nardone was convicted of smuggling alcohol etc. during a first trial based primarily on evidence acquired through illegal wiretaps. This conviction was overturned because the wiretaps were not admissible as evidence. On the second trial, Nardone was convicted again on other evidence. This conviction was also overturned, because the prosecutors wouldn't have had this evidence if it weren't for the wiretaps.