The truth they speak of is that 1) People in positions of power are not necessarily right, and authorities are vulnerable to self interest at the expense of others, and 2) More often than not, there is more going on behind the scenes that we do not know about, and the official story is not necessarily correct.
Where the conspiracy theorists go too far is by giving those in power too much credit, while discrediting the masses as extremely stupid, and easily manipulated. So the theories can be a bit too farfetched. Like the 9/11 inside job, the secret illuminati with hidden symbols, reptilians, ancient aliens building pyramids, ect.
On one hand, there are truths, like the FDA is representing Drug Companies, 9/11 was exploited by the Bush Administration to further his Neocon agenda, Corporations control the Government, and the rich 1% control everything. But this is not new news.
Sure, the official stories for the most part are bogus, but its hard to actually take full faith in what they say.
The conspiracy theorists fall into a kind of pharisaism where anything that has influence on society or political power needs to be destroyed, while anyone who in anyway conforms is some dumb sheeple to them.
The 9/11, Sandy Hook, Boston Marathon, 7/7 and other recent CT's are a symptom of a nation's inability to accept the cold hard truth of Islamic terrorism and a country that does not enforce measures that already exist to keep guns away from the criminally insane. The terrorism CT's are the worst of the lot, because they play right into the hands of our enemies because the rhetoric is basically the same -- that America is the root of all evil and that there are always Jews at the core of these "theories". It is the modern embodiment of anti-Semitism, so it's no wonder that these CT's are also extremely powerful in the Arab world and Iran. We should. Know better. Regarding UFO CT's we're shooting ourselves in the foot. The more we buy into the woo and conspiracies surrounding UFO's, the farther we get from answering the big questions with rational science. Conspiracy theories are the marking ignorance, paranoia and stupidity .