Knowledge is also very dangerous, and can potentially bring harm to the City.
Think of the City as a tribe -- everyone there has a certain way of life, of thinking, and even of what they consider is morally right and morally wrong. What they think of as right and true for them is the thing that has kept them alive in the face of their enemies, thus it must be always followed in order to be safe and protected.
Or, more broadly, think of the Imperium of Man of Warhammer 40K. Any sane person would agree that ordering Exterminatus on planets that have a minor cult which deviates just a bit from orthodoxy would be both overkill and extremely excessive, not to mention a breach of human rights.
However in WH40K such cults, no matter how minor, usually turn into a Genestealer invasion that cuts off the entire planet from the Imperium and lays it bare before an advancing Tyranid Hive-fleet, or into a Chaos invasion that swallows the planet into the Warp and dooms billions to unimaginable pain and suffering at the hands of daemons -- not to mention that whomever got offworld before that could still carry the taint and spread it like a disease to other planets. It's why the Inquisition and the Space Marine chapters are both so xenophobic and fanatical about fighting heretics, mutants (psykers) and xenos.
Closer to home, think about the culture of another people and ask yourself, is this what I want for my own home? Would another religion, another way of thinking even, be culturally acceptable to folk from a place like the American Deep South?
New things are treated as dangerous, both in the extreme fictional example of WH40K and the more ordinary examples of the Deep South. It implies change, and such change is not always for the better. This is why the Speaker exiled Osiris. In fact, here's his reasoning:
I admit, I found your questions divisive and disloyal, and I feared you might be capable of breaking our unity when the City's position had grown so tenuous. Why divert attention away from the Traveler, our only hope?
And then it got worse, dabbling in thanatonautics, Ahamkara-lore, chasing after Xur and the tricks of the Nine. Launching expeditions into the Reef and beyond at a time when ships were irreplaceable. Your quest split Guardians along ideological lines. This was your greatest crime: Hunters chose to pursue your visions instead of protecting refugees, Titans assembled teams to chase the legendary Vault of Glass instead of striking the Fallen, and Warlocks turned away from the study of the Traveler in favor of your ultimate obsession... learning the exact nature of the Darkness.
When debate became argument, and argument became acrimony, I realized you had already become a cult of personality, attracting Guardians who wanted a clear idea of why they were fighting, what they faced, and how they would ultimately win.
I fear you have become as obsessed with the Vex as Toland was with the Hive.
Toland, as we all know, had become a little too enamoured with the Hive and their philosophies, and the Sword Logic is, from our perspective, completely immoral, illogical, and antithetical to the Light and cooperation. Humans naturally tend to congregate and cluster into tribes, cooperating with one another; later on we formed cities, then civilizations, based on a common law and justice system. The Hive don't believe any of that. Might is right. The Queen of Armies is the only queen.
What the Speaker feared was the change of the City's thinking. As you know, Osiris had diverted the Guardians from what was important -- defending the City, protecting refugees, studying the Traveler -- to what was unimportant -- studying the nature of our enemies, to understand them as Ender Wiggin understood the Formics, and use that knowledge to destroy them. What was worse, they could have shifted away from what made them human and into something of a monster. I refer you to Toland.
Knowledge is power. But in a fight for struggle and survival, where everything is against you, knowledge is dangerous and heretical, especially from the wrong sources. And like in WH40K, knowledge from the wrong sources can destroy you and everything you have worked so hard to build and defend.
Perhaps what drives a Warlock to madness is truth.
(Witty quote, FTW.)