There's so much open to interpretation, when one says "self-defense". If you've trained in, say, sparring or serious kata, then you would be expected to breathe under pressure - rather than, say, like all new karate students, holding your breath under stress. Didn't we all do that the first time testing or competing? You would also hope someone trained in the classic MA would move about, and not stand still. These are lessons hopefully learned in most martial art training. Then there are the specific block and strike techniques, or take downs, probably pretty similar but certainly with variation (in efficiency) by style and/or instructor. How about some non-specific feeling of how to think and act during a fight? For instance, in boxing, you might be accepting of getting hit, albeit with a good blocking and protective technique. The same might be true of some traditional martial arts, perhaps deriving from how each point sparring philosophy works. Stupid, maybe, but true. So, in some styles, point fighting to a time limit with no limit of points scored, and maybe, little or no emphasis on defending oneself. However, in real world fights, I always - emphasize always - expected and attempted to avoid getting hit at all costs. I was not willing to trade techniques. (I got hit enough even with this attitude) So, analogous to the Japanese centric "one punch, one kill" feeling, of say, shotokan sparring with an Ippon win of a single strike. Certainly we have all seen point sparring which rewards stances, techniques and strategy which would be disastrous in real life.
I see some comments about avoiding a fight being some kind of MA trained response. Probably true, I think, but very limited in application. We hope our students would walk away from a senseless ego-based fight, but of course, in real life, walking away is not usually an option if you didn't instigate the issue in the first place. Also, there are those for whom avoidance is simply not an option, law enforcement or the military, for instance. Personal violence in such is not just unavoidable, it's part of the job description.
I was on a police department in a major urban area, with a high crime rate and a poor inner city. For the first 20 years or so, I did Judo. I picked just a few techniques which seemed to work for me, and practiced them all the time, from different positions, etc. Rarely, I might do something different from the Judo playbook, instinctively, and I used to surprise myself when I did. I had two or three strikes (palm heel and ridge hand my favorites), two or three takedowns, wrist-bars, sweeps, and they made up 90% of my fighting. But I also think the intangibles, breathing, movement, clarity of thought, learned on the dojo floor were invaluable too. Toward the end of my career, I switched to karate, but can't really say I used much of it in a real-world sense, because I was older and not in a position to encounter personal violence very often. I should also qualify this as admitting I used a blunt instrument rather than my fist whenever I could. So, nightstick or flashlight to the head or body...not really a traditional martial art.
Last, you can't train for martial spirit. One has heart, or not, However, students can - and do - learn that getting punched in the nose is not the end of the world, that a little blood won't kill you, and that usually, mental toughness can be the defining factor in a fight. Well, some fights anyway.
So, I agree that traditional martial arts can lead to an ability to defend oneself. Some styles may lend themselves to more efficiency than others, and certainly, there may be a few styles - or more likely, some instructors - whose training could lead to a disaster in the real world. But, if the question is, does traditional martial arts training make you better at defending yourself, how could the answer be anything but "of course."
