actually some spinning kicks can be used, look at goerge st pierre. spinning back kick. he pulls it off every time. also an amazing guy from the old days, sugar foot peter cunningham trained with beny the jet.he he was a thai boxer who originally came from a traditional kicking style like tkd . that guy used to kill thai boxers. but he was a thai boxer with amazing tkd form. so again tkd can have some good tools but needs to be incorperated into a better fighting style like thai etc.
ps, i have no interest in tkd. im not a fan of it for fighting. i used to do it along time ago, and have moved on believe me. but i just want to be fair and say that some of the kicks they teach can be used in thai and mma. but only when its used by someone who also knows thai boxing / mma etc.
another example was a guy who knocked out maurice smith in k1 along time ago. he was a very good dutch /thai boxer and used an axe kick. maurice never saw it coming.