Greystoke, you make a valid argument but you are wrong. It is not acceptable to say to a court "If you knew what I knew, then you would find this man guilty."
"What do you know?"
" I cant tell you its secret"
The last time I heard this used was by Tony Blair, turns out what he knew that we were not allowed to know led to the Iraq mess. If you have wire tapped a suspect legaly present the evidence in court. One could get the feeling that the reason evidence cannot be presented to a judge is on the grounds it is bollocks and agencies are not above trying to avoid looking stupid and heavy handed.