However, for as much as he is worthless as a witness, he is twice as valuable as an informant. Remember the old saying..."knowledge is power."
The FBI under Hoover used to follow that dictum. They'd accumulate evidence illegally (usually through electronic surveillance that wasn't court-sanctioned), then use it to go after perps and witnesses. They'd confront them with what they'd heard without identifying the source or its illegality, then get them to plead out or turn rat. But, the occasional (and I mean very occasional) defense lawyer who challenged them to produce the evidence sometimes made them back down.
