Oh, boy. I get into this discussion often.
Martial arts has almost always been developed for combat use. They are fighting systems at their core, pure and simple. If you want to defend yourself, you need to train yourself to do just that. That means fighting "dirty" is on the table. You learn to use whatever gives you the advantage over your attacker.
If you train for sport only, you are teaching your mind and body to fight according to the rules of that sport. You are also training yourself to engage only one attacker.
Now, let's say you train for self defense only; targeting the knees, eyes, etc. These tactics are of little use if you do not know how to engage your attacker(s) (knowing how to fight). This is where a sparring partner(s) comes in. This is where you engage in "sport" martial arts in order to learn how to get past your attacker's guard, get your attacker off balance, or how to spot various targets, and how to position yourself if you have two attackers, i.e. not being in between them.
Having said all of that, martial sport can be a useful tool. You just have to throw the rule book out. After all, if you train only for competition, you'll defend yourself like you're trying not to get disqualified and win a match. If you train to save you life or a loved one's life, you'll fight like it.
That's my opinion.