And, even if the FBI could know that a hitman left a series of incriminating phone calls without already having evidence which could be admitted against the defendant of the content of those phone calls, a wiretap based on a warrant issued after the fact wouldn't recover those contents.
Further, I don't think anyone in the thread has argued that wiretaps aren't useful in criminal prosecution, the issue which started this was a poster expressing that it would be wonderful if wiretaps were categorically outlawed, which does not require believing that they aren't useful in certain circumstances. It could, for instance, proceed from the belief that the capacity is so likely to be abused covertly in circumstances where it is inappropriate if its existence is tolerated at all, that the usefulness it has in the circumstances to which it is appropriate is outweighed.
Nobody is arguing against that. What people are complaining about is, among other things, the bulk collection and retention of records; the gathering of records upto 3 degrees of separation from a suspect; etc.