For me it was more about modulation than the ability to lock up a wheel (which I hope to avoid), and the greater confidence it gives me on prolonged, steep descents, as well as reducing hand fatigue from prolonged braking.
They are more expensive, more finicky to adjust and maintain, and in my case at least, I chew through pads and rotors fairly frequently, and the cost adds up.
If you have carbon rims, they are probably almost a necessity. If you ride off-road a lot, they are very reassuring to have. If you ride under wet conditions frequently, they are probably a safer option.
But remember, Occam's rotor can slit your throat.