I'm a novice in taijiquan, but used to train Bujinkan Budo Taijutsu (Ninjutsu) before. Many people claim the art is internal, and I don't think there are any students from taijutsu having themselves in UFC, or or any kind of competition the MMA people usually go to.
I had and still have some friends who went to MMA and BJJ kind of arts while I trained ninjutsu. They wanted to train and spar with me and sometimes we tried...the problem was that with their rules, they could often beat me. But how do you even try to "spar" with a person who comes from a sport art where there is an aim to "win"?
The art I trained, was deadly, the katas were short and meant for real life situations were you really are in deep dren. Having to fight many opponents same time forces you to forget every kind of kick or punch or "shoot" (from BJJ) that are typical for MMA people. Instead a kick to the balls, knees, an open palm strike to throat, to eyes, using your nails, any kind of tool you can find from your pocket to get out of the situation alive, using the opponents clothing against him...were the kind of things that were trained.
Now how do you compare these kinds of arts,
martial arts to MMA where there's the simple (and for my opinion, stupid) aim to get the opponent knocked out in a ring (with no real danger), or having him in some sort of BJJ -lock on ground (which would mean on street, the opponents friends come and kick you in the head)
And how am I supposed to try and compare my art against such sort of people? "Whoops, I just dodged your punch and stuck my middle finger into your eye and turned you into an eunuch with my left knee strike to your balls." The students of internal arts usually understand this and avoid any kind of competition or place where people pump themselves up in gyms to get a steel kind of (clumsy) body to be undefeated...in a ring.
External MMA fighters might have their time and highlights when they are in their best ages and get the glamour of being the king in the ring with rules...but when they get older and they notice they no longer have the time, effort and possibility to keep themselves as fit as they need to, for doing their art, an internal arts trainer still trains and learns the applications to the real life situations.
Another thing about MMA-arts...knive, baseball, or the worst, a gun assaults? MMA-fighter will be in trouble when suddenly their ring meets the real life, the real streets, and their "philosophy" of beating the other one up and getting the ego boost serves no good. I would have felt so sorry seeing an MMA-fighter to really test his skills against the teachers in ninjutsu, and I believe the same goes for taichiquan, baguazhang etc etc.
Why doesn't this usually happen? Because the internal arts masters usually don't have this idiotic ego-need of testing their skills against MMA-people who train to win, not to learn life. And perhaps, in a ring, with the rules, with the MMA-fighter getting his/hers proper warm-up needed they would still get the internal marts master down with a knock out...so what? Doesn't proof anything else than that in the ring they have their time.
Here are also a few examples...
http://www.youtube.com/watch?v=xQFFpA0C4LU <- kung fu against mma
http://www.youtube.com/watch?v=3W_zqK635dQ <- viewpoints against a grabbler.
http://www.youtube.com/watch?v=0W1ym3yggR4 <- undisputed evidence of tai chi being a perfect art even inside a ring.