What if O'Keefe has evidence of wrongdoing between ACORN and Landrieu? After all, ACORN is based in New Orleans.
It would have to be VERY compelling evidence for a judge to allow a jury to hear it. Even then, one crime doesn't excuse another, so it's relevance to their guilt or innocence is suspect, if also possibly beneficial at sentencing.
There are PLENTY of cases where FBI agents broke wiretapping laws on some very bad people who I'm sure that were guilty of some very bad things, but that doesn't inoculate the FBI agents from the crime of an illegal wiretap.