Fights by their very nature are struggles; struggles between two opposing forces each trying to impose their will on the other. Both combatants are fully animated, able to do whatever they want to making them highly unpredictable. Fights are made up of momentary, fleeting opportunities that appear and could be exploited but if not, disappear as quickly as they surfaced.
Do a Google search and watch a video of the double rod pendulum. The movements of the pendulum are erratic and chaotic. They can't be predicted. Each new movement is the result of an equally unpredictable input. The double rod pendulum is analogous to a fight.
Systematic approaches fail in fights because any group of techniques that are predicated on reliable anatomical responses are unrealistic. You can never know what an attempt to hurt or make someone unconscious will result in because there are simply too many variables. Are they high? Drunk? Do they have a chin? Are they muscular? Adrenalized? Do they have skills? Are they familiar with fighting and getting hit? Are they fearful and amped up?
Systems fail because fights are not systematic. Systems evolve not because they're effective for fighting, they evolve because they're effective for teaching. The linear nature of systems make sense to people who want or need an absolute answer or guarantees for techniques. Yet there are none. Learning a system is only effective when fighting someone else in that system but when faced with chaos--a person whose movement is unrestricted and unpredictable--they fail miserably.
There are two ways to deal with the chaos of an attacker. The first is through true counterpunching. Training yourself to immediately see and capitalize on those momentary and fleeting opportunities (vulnerabilities) that are created when a lesser fighter attacks. A boxing example is slipping a jab to the outside while simultaneously using your straight left to the point of the chin. A combatives example would be to hand hook someone who grabs you as you throw the upward elbow to the point of their chin. In both examples you are protecting yourself as you attack from their initial and subsequent attack. You're protected from the jab by head movement and the follow up cross by being outside as you throw back. In the combatives example, you're checking the attacking hand and accounting for it and protecting yourself from the punch by using the upward elbow effectively blocking anything.
Both examples are true simultaneity, one of our combative principles. By adhering to principles instead of relying on techniques or a systematic approach you avoid succumbing to chaos.
The second way to deal with chaos is to make it stop; when attacked, immediately attack and make the chaotic attacker unconscious or damaged to the point they either can't or won't keep fighting. In terms of the video, if you want the chaos to stop, smash the machine so it can't operate. Training to immediately turn predator into prey is absolutely essential to what combatives is.
Focusing on both of these approaches requires you to "spar." Not to just apply techniques on a non-dynamic training partner who may paw ineffectively at you to initiate an uninspired systematic response. Yes, we all have to learn technique but equally (if not more) important is the ability to apply those techniques extemporaneously and spontaneously; otherwise whatever you're doing is just another expression of kata.