While developing an unarmed combative s arsenal, it's important to understand that your ENTIRE body is comprised of useful tools. Training with no limitations, IE reality based personal protection, should encompass ALL of the tools available to you. However, bear in mind that not every strike is a debilitating knockout blow. For example, using your lead hand or lead leg to strike is often not as powerful as a rear hand or leg strike, but it is often less telegraphic and can be used to set up finishing strikes.
The body has other useful, often overlooked, applications as well. For example, the shoulders can be used defensively to protect your chin, used for "grinding" while engaged in a tactical ground fighting situation or used to create space during a situational self-defense application such as a choke from behind. In extreme close quarters, spitting or biting could also effectively be utilized to create space or cause an involuntary reflexive response due to their psychological effects on a person's natural aversion to such things. Creating this space or involuntary reflex can put you in a better position tactically for follow up measures.
While punching with a closed fist is functional and has its purpose, some have argued that there is greater versatility when using an open hand. Using such tactics enables a person to not only strike using the palm, web of the hand or "knife" edge of the hand, but also grab (hair, ear, groin), finger jab/poke (eyes), tear (throat, "Fish Hook" the mouth) and even pinch to cause an involuntary flinching response. When using various weapons to strike offensively, primarily the hands which contain some of the smallest bones in the human body, keep in mind proper tool usage as it pertains to soft tissue (body) v. hard tissue (face/head) targets.
Another aspect of full body usage is tactile sensitivity, particularly within grappling range or during a clinch. In terms of combat, tactile sensitivity refers to using your body's sensitivity to feel an attacker's movements (or even weight shifting) without looking directly at the attacker. An example of this would be using your legs to "vine" the leg of the attacker during a bear hug attack from behind to prevent the attacker from lifting you.
Remember, your reactions during a stressful encounter will be dictated by the environment in which you train. Learning to use your entire body against an attacker is another advantage of training in an eclectic fighting system like Krav Maga.