Ledow raised the issue of a case where the police use the tapped phone conversation in their investigation and then find solid proof that the defendant is in fact guilty. Just because the police may not have looked there without the illegal information doesn't change the fact that they can now prove beyond resonable doubt that he is in fact guilty.
But it's still illegal.
What you are saying, effectively, is that the ends justify the means. Unfortunately, once you start down that road, even a little, you can justify all the acts that I pointed to in the previous post. I mean, if the defendant lied about his alibi, he must be guilty, right? And now that you know he lied about his alibi, you can apply pressure to the person providing his alibi (without telling them why you know the alibi is false), who will admit the falsehood, and you can now show in court the the defendant lied. Coupled with the other circumstantial evidence, that's easily enough to get a conviction... Of someone who is innocent.
I know this is a very TV drama sort of example, but these things really do happen all too often. We have these laws in place for a reason. They are not there just to hamstring the prosecution, they are there to protect the innocent. As soon as you start saying it's ok to break the rules since the outcome is positive, you are also accepting the fact that it's OK to send a few innocent people to prison in the name of catching the bad guys. Unfortunately, there is no happy middle ground in this case.