You are correct in part, it but also incorrect in other parts.
It is correct to state that any evidence used to prosecute an individual must be legally obtained. But the question (in the present case) then becomes, when did the warrantless intercepts change from being legal to illegal and the answer to that is found in the "primary purpose" test.
The Truong court, who is credited with the genesis of the primary purpose test, concluded that while the President may lawfully conduct warrantless surveillance for the purpose of gathering foreign counter intelligence, it would NOT be similarly lawful for the government to do so when the primary purpose of the intercepts has changed to commencing a criminal prosecution of the targeted individual.
Therefore, when I stated that "none of it can be used directly in a criminal prosecution", that was a true statement because the evidence Cboldt was referring to was gathered through warrantless intercepts solely for counter intelligence purposes. The moment the intent of the surveillance changed to commencing a criminal prosecution, the government would be obligated to obtain a warrant in order for the intercepts to remain lawful, and the warrantless evidence gathered prior to that point, would be excluded, under the primary purpose test.
Outside of their intelligence value, the only use the warrantless intercepts could play in a criminal prosecution, would be in drafting the probable cause affidavit, necessary for obtaining a criminal warrant.
The primary purpose test is dead. If the intercepts don't violate the 4th Amendment or FISA or the Wiretap Act, there is no legal ground for excluding any evidence obtained.