As I talk with a lot of coaches, physios, and physicians a phrase I often hear is: "well he/she just needs to get stronger". Even those supposedly "in the know" say it so much it's almost reflexive — as if more strength or capacity is all that is preventing someone from getting healthier, picking their kids off the ground, winning gold medals, etc. Often we are more focused on the rep completion: rep or no rep, rather than the process or competency of that rep.
I think the most pertinent thing here is defining the goal of training. For nearly everyone, including professionals, the purpose should be to enhance movement efficiency and adaptability by teaching then reinforcing motor patterning (competency). Only then should capacity (output: numbers/stats, times, weights, reps) be pursued. Sure, a competitive weight lifter's (Olympic and power) main goal is maximizing capacity — the more weight, the more capable. In this group, it is essentially a linear relationship: the more weight the lifter can move, the more competitive they are.
But what about everyone else, from pro sports to weekend warriors and soccer moms? Does more strength automatically equal better performance? Injury prevention? Increased longevity? Does an NFL running back who squats 450 but keeps blowing out his hamstring need to "get stronger"? I guarantee if he gets stronger and squats 500 his hamstring problems will not just go away. The teaching and reinforcing of even basic motor tasks is almost always lacking and has probably been skipped past or brushed aside since junior high gym class. I am all for getting bigger, faster, stronger — these are crucial — but not at the cost of sustainable movement.
Yet this capacity-first mindset is easy to fall into and the long-term cost can be quite high. It's easy to throw more weight on the bar and blow up your ego, it is much more difficult to tease out movement faults and restore movement variability - I can say this firsthand as I rework my own competency in Oly lifts. But I think this argument for competency first fits both the biomechanics and neuroscience silos. Loss of movement expression, especially under increased load, only serves to stress and threaten the nervous system contributing to the experience of pain and plateau or loss of performance. A stressed-out system lacking neutrality and variability is a rigid, painful and injury-prone, not-gonna-last-long, system. Good riddance.
There needs to be a baseline competency that we must obey and display prior to going after capacity. AND, in the pursuit of capacity, we continue to monitor movement fluency, efficiency, and adaptability so that competence is maintained. I have found that maintaining competency allows capacity (the outputs) to come more easily and without as much risk. Look at Shannon Turley's training program for Stanford Football - quality is sought over quantity and ego is left at the door.
What is the best measure of competency? I don't think the system matters as much as the pursuit. You can make an argument for the FMS or anything else you can come up with that shows you (as the athlete, coach, or physio) where you stand and where you need to go in order to reach the training goals. It should be specific to the individual's training goals and should be revisited often. Those who last the longest (in sport and function) are often not necessarily the strongest but the ones who move with variability, efficiency, and resiliency.
Respect the movement process.