Warning: SPOILERS for Justice League Odyssey #21
The MCU's the Masters of the Mystic Arts. The Ancient One considered herself to be this reality's sworn protector, using the Time Stone to peer into the future and identify potential threats.
"I've spent so many years peering through time," she told Doctor Strange at the moment of her death, "looking at this exact moment. But I can't see past it. I've prevented countless terrible futures. And after each one, there's always another. And they all led here, but never further." Doctor Strange proved the Avengers: Infinity War, the future Sorcerer Supreme put the Time Stone to the same use as the Ancient One. He explored over 14 million different future timelines, identifying the one where Thanos could be beaten. To Thanos' surprise, Strange then fails to use the Time Stone for offensive purposes. "Yet you never once used your greatest weapon," the Mad Titan observed.
The truth is that Doctor Strange - and, indeed, most likely the Ancient One before him - wasted the Time Stone's potential. This fact is ably demonstrated over in DC Comics, where Dan Abnett's Justice League Odyssey has revealed just how powerful a weapon temporal manipulation can be. The book features Epoch, the self-proclaimed Lord of Time, who has developed absolute mastery of the timestream and even the Multiverse itself. Unlike Doctor Strange, Epoch uses this power to devastating effect; he moves people and objects through the timestream at a whim, transporting them to the past or future. This, coupled with his intimate knowledge of time, means he can lay complex four-dimensional traps for his opponents. It would also mean he could deal with Thanos with staggering ease, simply moving the Mad Titan forward in time by a millennium or so.
One of Epoch's most fascinating strategies saw him manipulate his own timestream, pulling past and future versions of himself into play, and even alternate-universe iterations. In this way, he can assemble a small army of Epochs, all powerful and potent. There are risks to this gambit; if a past version is killed, Epoch needs to swiftly figure out how to repair history before he simply ceases to exist. But, again, imagine Thanos confronted not with one Doctor Strange - but with an army of them, plucked from different points in the Multiverse, all wielding the same unfathomable power. Even the Mad Titan couldn't have handled that.
So why didn't Doctor Strange use the Time Stone in this kind of way? The most likely answer is offered by Doctor Strange itself, because it suggests the fabric of time is very vulnerable indeed. In one scene, Wong and Mordo saw Doctor Strange experimenting with an apple, and they were outraged. "Temporal manipulations can create branches in time," Mordo warned furiously, "Unstable dimensional openings. Spatial paradoxes, time loops!" If just playing with an apple risked breaking time, unwary use of the Time Stone would have the potential to tear it apart. Disturbingly, the MCU subtly hinted this has actually been done before; the Masters of the Mystic Arts kept the ancient Book of Cagliostro, a sorcerer who had experimented with the powers of the Time Stone and had learned wisdom the hard way.
That, presumably, is why the spells were always written before the warnings; because Cagliostro came up with an idea, jotted it down, tried it out, and then found out just what could go wrong. No doubt Doctor Strange was not keen to repeat that process of discovery.
