Kendo is pretty damn useless in mma if that's our metric.
Thread started off with this great response, and more followed.
This last weekend was testimony to the worth of techniques not permitted in the cage. Where the effectiveness of a simple (though inadvertent) eye poke was on display.
What MMA has done best educationally, is drive home the fact that if you cannot ground fight, you are at a serious disadvantage against an opponent that is skilled at it. But as the quote above illustrates, it should not be the sole criteria you judge a MA by. Eye pokes, fish hooks, small joint manipulation, groin strikes, and attacking the back of the head and cervical spine are illegal for good reason. Hell, even stomps, soccer kicks, and knees to the head of a downed opponent make for a drastically different fight, as we saw in the days of PRIDE.
Chinese martial arts and TKD are FAR from worthless. But, there are places where they train them so poorly, and unskillfully, that one might mistake them for such.