You could see things a bit biased when you practice a certain way but realistically a mixed martial artist is going to train and either make it or break it. Meaning what does he do when he realizes he has no chance of being a contender, been continually busted up over a period of several years, will he continue being his classmates UKE, how many will continue to be able to train. I personally know someone that was punch drunk by the time he turned 22. He started training when he was 18.
I'm not saying which is better, not at all! I'm saying if you plan to train for the rest of your life (like a true martial artist), then you SHOULD find a mix that works for you. The number of techniques that are most "effective" are the same techniques that all mmartrists are learning. IF THEY ALL TRAIN THE SAME, THEN YOU NEED AN EDGE, THAT IS YOURS AND NOBODY ELSES. You do that by being exposed to more than what your competitors are training.
The problem is you need a specialty area. So say you train mma and every other day go to a bjj school, thats a good mix that rounds out all fighting ranges and you will continually learn new techniques.
It dosen't have to be bjj in the mix. Where you take your art is your buisness. What I'm saying you need a mix of all ranges and how you get it is your journey.
My method of teaching is to teach the arts separately and together. The reasoning is all techniques need to be understood. Have you ever heard the term "position before submission," its a very true statement for the grappler but your postion isn't any good if you don't "understand" it. With a limited understanding you'll be passed, sweeped, pinned, submitted, knocked out, whatever, it all leads to "exposure" to techinque. If you want to add new variations "to your mix" that the other schools don't have, then you need more exposure to technique that you can adapt and modify as needed.
I'm not saying any schools out there are not teaching in a way that their students don't "understand." I'm validating the way I teach. If you learn your arts separately you will find techniques that work for you that others don't like, you will be exposed to more and you will have a deeper understanding. But you also need to develop a flow between your arts.
THE NUMBER ONE MISTAKE IS LIMIT WHAT YOU PRACTICE. BOTH MMA AND TRAD. ARTS MAKE THAT MISTAKE!