Real opto means that the signal side (3-pin plug) is completely electrically separate from the power side (battery and motor). In fact, even the ground should not have continuity between the signal ground and battery ground. This is true optical isolation. It completely eliminates any and all conducted electrical noise from the power system. IMO, this should always be the preferred setup, and used whenever possible, with zero downsides. In this case, the power wire from the flight control to the ESC is absolutely mandatory, as the processor in the ESC is dead without it.
However, it must be known, that the great many "opto" ESC's for multirotors are fake. They are not truly Opto. The ESC circuitry is powered by the battery, and the ground has continuity straight through. The only thing they have done is eliminate the BEC circuitry.
There is very little benefit to these systems, save for the dubious advantage of not having to leave the red wire from the signal cable disconnected.
