While it’s especially beneficial for effective dissident groups to spy on the CIA in order for them to not be exterminated (so easily), the average American benefits from it as well. That way we don’t have to wait decades for some heavily redacted document to be released which talks about a few of their activities.
There is no “congressional oversight” of the agency and even if there was there’s no “popular oversight” of congress. There’s no respect for democracy or the American people within the CIA if their actions are any indication, therefore there’s no need for lovers of democracy to respect their desire for secrecy – a secrecy that works against the interests of Americans.
All other secret government bureaus should be spied on as well of course, such as the NSA, Homeland Security, and the FBI.
Then when any action they take is against American democracy at least the people will know about it (ideally beforehand) and can take maximum action to stop it.