Flaws in your squat technique will hold you back from progressing. Overhead squats can improve the squatting pattern and should be a regular part of your strength training program.
The overhead squat requires a more upright posture than the back squat and you're forced to execute the squatting pattern precisely, otherwise the bar would drop to the front or back.
Have a look at the video. You can see the high concentration level of the athlete and accurate execution.
The Fortress believes there are four fundamental directions a weight must be pulled or lifted in order to build a kick-ass back. He calls it his Directional Back Training Principle and it has nothing to do with North and South.