Hi,
You know, I'm going to go the controversial route again. Let's see how this flies...
I have come to the realisation that no martial art or sport is "good" for self defence!
That's right, none of them, not even my own. Neither is MMA, Tae Kwon Do, or anything else that gets the moniker "martial art" or "combative sport". They are not designed for it, they are not geared up for it, they do not have it as a goal in any way whatsoever. Does that mean that the skills learnt there cannot be utilised for self defence? Not at all.
As I've said in another thread, the idea that martial arts are good for, or even meant for self defence, is rather inaccurate. It comes from marketing and popular media, not from the systems themselves. To understand that, you simply need to look at what the systems themselves actually are, what they are teaching, and how they are teaching to realise that. Then you need to understand what self defence actually is. That difference will be the focus of this post.
To begin with, martial arts. And by that, I am refering to what is refered to Traditional Martial Arts, whether newer systems such as Aikido, Tae Kwon Do, Karate (in the main), Judo, or older systems such as Japanese Koryu arts, Chinese arts such as Wing Chun, Hung Gar, Taiji, Bagua, or similar. These systems teach methods removed from the legal requirements of todays society, and against attacking and assaulting methods not found today, including against weapons that simply are not encountered.
The way things are taught is a formalised structure, learning skills by rote, as it were, and then methods of application according to the methods of the system. To take a few examples from the Japanese Koryu traditions, a system such as Katori Shinto Ryu will only ever train with pre-structured series of techniques (kata), with the speed, intensity, power and so forth being increased with time. There is also a great focus on study of things such as strategy. Systems such as Owari Kan Ryu (a school of spearmanship) actually start with shiai (a form of competitive sparring) before kata training. And systems such as Araki Ryu have a tradition of heavily pressure testing kata. So there is a variety of teaching and training methodologies just in that small area. But you may notice that the focus in none of them is self defence.
The more modern systems such as Karate often talk about their self defence aspects in their teachings, but is it really there, in the actual art itself? Again, teaching happens primarily via repetition of basic movements and kata, often with sparring used to heighten the application of the skills. But sparring is not self defence application, as it is a method of sharpening the practitioners skills against another practitioner, often of similar skill and experience, using the same strategies and tactics, attacking and defending with the same techniques. This is very different to self defence, but we'll get to that.
When it comes to sport systems (which can be part of traditional systems, such as Karate, Judo, Tae Kwon Do, and so on), the sporting aspect is really a method of testing individual abilities in an even setting. The method of testing varies, obviously, but it is really just variations of the same idea. In order to test safely, there are always rules involved, both explicit and implicit. As the theme of this thread is MMA, let's look at that.
The explicit rules are fairly simple. Certain techniques are not allowed (fish-hooking, eye-gouging, groin strikes, in some organisations elbows and knees are disallowed in certain circumstances, such as when an opponent is on the ground, or to certain targets such as the back of the head, and so on). The implicit rules are where we get our biggest deviation from anything close to self defence, or, as many put it, "a real, or street fight". These include the environment (padded floor, cage or ropes surrounding it), single opponents, a lack of weapons, no "surprise" attacks, and so on. Added to that are the methods of "winning", which include ways os scoring points, or gaining submissions (implicit rule: don't actually break anything or damage in a permanent way), as well as ways the fight will be stopped (the opponent gives up, is submitted, or gets knocked out, the time expires, the corner throws in the towel, and referee stoppage). These are important, as they again take you away from a real fight, especially the referee stoppage.
There are other major differences, including the timeline for both, but I've covered those often enough in other threads. The point here is that these methods are again not in any way methods of self defence. But that brings us to what exactly is self defence anyway, and how is it taught if not in martial arts or combative sports?
Well, the first thing to realise is that self defence is not focused on techniques. It is, instead, focused on protecting yourself and others against outside (uncontrolable) dangers. So your first port of call should be to recognise what those potential dangers are. And they are very different to what you encounter inside a ring, cage, dojo, kwoon, dojang, and so on. Added to that is the fact that the responces from a self defence approach will be different to that of martial arts or combative sports, in that self defence will seek to keep you safe in the easiest way possible (avoidance of situations that lend themselves to danger, followed by de-escalation, followed by pre-emptive striking, then you start to get into actual defences against attacks, but even then it's based around easily adapted principles rather than techniques), whereas martial arts and combative sports will seek to engage (in most cases, a very few older systems will teach escape and avoidance as well, but the majority will still be focused on engaging), and have a desired outcome of subduing, defeating, injuring, killing, or other, but not escape safely. This is the biggest key difference between self defence and martial arts or combative sports.
When it comes to martial arts and sporting aspects, if these are taught under the guise of self defence, without any consideration to the realities of what self defence actually is, it simply is not self defence. The things learnt there can certainly be very useful in a self defence situation, however it is not what they are designed for, and that should always be recognised above all else.
IMHO, I think that many TMAs can benefit from MMA, and likewise, MMA can benefit from TMAs. MMA will give you the contact, the striking, kicking, clinching and grappling. But like anything, it too has its limits. Keep in mind that people need to be able to seperate the ring mentality from the real world mentality. 
