MMA training can be very useful. However, the focus of MMA training is a one-on-one sporting competition where competitors are unarmed.
What's lacking in MMA, for example, in dealing against weapons....is many folds lacking in SD and TMA, which is sparring hard for full knockouts to really test your chin and ability to fight back.
Training to disarm knives, guns, etc. in general, is just fantasy role playing. Most instructors themselves are basing their teachings on theories also w/o any real life experience. I've trained with weapons in a Jujutsu school for about a year. Instructors were 2 cops, so unlike most SD instructors, they've had some REAL experiences vs. knives in their line of work as cops. And they weren't Barney Fife-type cops in the sticks neither. These were Washington DC cops, with one working in Southeast DC (really bad area on par with Detroit, Oakland, Baltimore, etc.). And what I've found out is that during sparring w/a rubber knife, I can easily slash the hell out of them using my footwork, hand speed, agility, etc... all from being a trained and experienced as an MMA fighter. At least 8-10 slashes before they get frustrated and go full on, banzai-suicide blitz to get slashed some more, but succeed in grabbing my knife hand to disarm me. They would never do such if my knife was real and they've got 8-10 deep gashes with their own blood squirting all over the place. And I wasn't even stabbing them neither, because that rubber knife is real solid and can take an eyeball out, thus I was being extra nice.
It's a good thing that not many SD students are attacked for real, so they never get to find out if what they've been training all this time is worth anything.
It will toughen you up, get you in shape, and if the school is good and you train diligently and well, you will see excellent results. It's a great option. However, it's not the be-all end-all if self-defence is your goal.
MMA certainly isn't an end-all system for SD, but it surely beats most SD systems out there IMO. Knife on knife, my experience as a fighter would mean that my hand speed, footwork, reflexes, etc. is going to be way faster than someone who never fought before in their life. Just put the knife in the Boxer's or MMA's power hand and reverse the stance. The jab hand is now going to be lighting fast and powerful. This is exactly how I train with my knives, I shadowbox with it.
A Muay Thai clinch is unwise if the opponent has a knife you don't know about, for example. If he pulls it, you won't see it, and you'll be in a dire situation without even knowing it, until it's much too late.
Well no experienced MT fighter would just jump in and clinch w/o seeing that there's no weapon first. But once the clinch is about to be locked (takes about 1 second), knees are already going towards their face. I'd like to see someone try to reach for a weapon in their pocket while in the MT plum and eating about 2-3 knees per second to the face.
However, if you take MMA, you'll learn how to punch, kick, and grapple, which are fundamental skills you can't do without. If on top of that you take a more SD-themed art (KAPAP is a good choice if you can find it), you'll be as well-prepared as you can be. However, keep in mind that no matter how good your weapon defence is, against an experienced weapons guy, the unarmed person is in serious trouble. You can give an athletic, agressive person a day's training in knife work and they'll make mincemeat out of most unarmed black belts. Would you fight someone whose jab can kill you? I wouldn't... it's bad math. But train as much as you can so you the odds will be stacked in your favour as much as they possibly can be.
That's why I don't leave home w/o my 9mm and at least 1 knife, but usually 2 knives and my EDC.