What Bas said
But is it effective at teaching you how to defend yourself in a close-quarters environment where anything can happen, and where the weapons used are beyond just hands and feet?
I have noticed this myself, and here is a real-life example of what happens when you try to apply MMA training to a situation to which it is not well adapted.
The other day my KM training partner and I were practising knife defences from a leg range at a more advanced level, where once you've stopped the initial attack you attempt to take away the weapon. Now, my buddy recently had his amateur MMA debut and had been training long and hard in jiu-jitsu and grappling, and he and I have been training together for a long time, so we're very comfortable with experimenting and goofing around a bit from time to time.
So he decided to try something new with the takeaway by putting me into a standing figure-4 armlock.
There were two huge problems with doing this.
First, even though he's bigger and taller than me- which is saying something, as I stand close to 180cm and weigh over 85Kg- he didn't have sufficient leverage standing up to make that hold work. So even though it hurt, a lot, to have my shoulder twisted around like that, I was still able to move around and slip out of it- and thereby move the (rubber training) knife to my left hand.
Second, and far worse, he got close to the knife.
A knife is a short-ranged, sharp-edged weapon. The absolute LAST thing you ever want to do is get close to a knife. And if you are ever faced with the prospect of a knife attack and you're not shitting your pants at the idea of it getting close to you, I'd suggest you go to see a psychiatrist, because there is something severely wrong with you. Knives have killed far more people in human history than guns have, and not just because they've been around much longer.
So it should come as no surprise that, when I was able to switch my knife to my left hand, even though my good friend had me in a standing hold, I could still slash and stab him.
In real life, he would have lost toes and skin- at a minimum. And that would be right before I shanked him in the groin. If that were a real-life attack, he might have torqued my arm around and caused me to slash myself near my kidneys- but he would have received at least one serious injury in return.
Worst of all, that injury could very easily have been a six-inch knife blade buried deep within his nether regions.
Which would have been a really bad way to remove himself from the gene pool, considering the kid's only 18. And which would have been REALLY awkward to try to explain to that hot nurse at the hospital.
And all of that would have happened because he obeyed the instincts driven into him from his MMA training, not from his KM training.
This isn't a new problem. There are BJJ schools that teach street defences against weapons where you will actually see the instructors telling you to go to the ground with the weapon- and roll with a knife near your body.
Personally, I think that's insane.
I love using knives; I pretty much always have one with me these days, ranging from a small short (but extremely sharp) Leatherman tempered steel blade clipped to my gym bag, to my Leatherman multi-tool, to one of two full-sized Victorinox Swiss Army knives in my pocket. But I also respect knives, which is why I'm very careful when using one. The slightest slip in attention, the merest hint of carelessness, and that knife becomes a lethal weapon that can be used against you, instead of a tool that will save your life.
There is nothing in the world wrong with training in MMA. If that is what you want to do, go for it and the very best of luck go with you. Just remember, it's a young man's sport; when you're a bit older (like me) and you've accumulated a bit of wear and tear on your joints (like me) and you're carrying around one or two or ten injuries (like me), then maybe you might be a little better suited to something where you don't have someone trying to tear your joints apart every time you roll with him.
That is where systems like KM and other similar arts come in. They are designed from the outset to be something that pretty much any reasonably able-bodied person can do; there wouldn't be much point to teaching self-defence courses if the techniques only worked for left-handed gingers over 6 feet tall.
They are meant to teach you how to use weapons properly; the best such courses might just take you out to the gun range and have you squeeze off a few rounds, just to see what holding and handling a gun actually feels like. If you live in an area where gun laws are really retarded (unfortunately, also like me), then the best they can do is make sure that you can train against realistic sticks and toy knives and toy guns, but it's still better than nothing.
Having said all that, if you put an MMA fighter up against even a highly skilled KM black belt, who would win?
Nine times out of ten, I'd give it to the MMA guy.
The only reason it isn't ten times out of ten is because I've seen what the best KM practitioners in the world can do; they categorically ARE NOT the kinds of people that even trained fighters would want to mess with.
As always, that is because MMA and KM are designed for two very different purposes.
MMA is designed as a fighting sport- one that is tremendous fun to watch, and one that is about as close to true no-holds-barred fighting as you can get without killing people routinely in the process.
KM is designed to be a system of combat, not a sport. Its starting and ending points are very different. Its aim is not to allow you to survive a 9- or 15- or 25-minute fight. Its aim is to make sure that, if shit goes down, you get in, you get out, and you go home more or less in one piece- while the other guy goes to the hospital (at minimum).
Choose what works for you and for your particular aims. Experiment, see what you enjoy the most. Stick to it. Train hard and smart, rest and recover plenty, and mix things up from time to time. You will become fitter, stronger, faster, and much more manly in the process- and you will make friends who will stay with you for life.