I disagree on much that has been said, especially about using kicks for self defense. Lets focus our observations strictly to the UFC since now so many have had a chance to view it and it's fighters.
I personally rarely see someone taken down after throwing a kick. This is why I brought up this subject, also because I am WTF TKD and trained to counter my attacks off of a kick, I see a lot of uskilled kickers in the UFC, just about every round kick is telgraphed and can be countered if the fighters had the right kind of trainnig. Kicing in the UFC and countering off kicks is an untapped resource. Fighters allow there opponenets to throw giant hay maker round kicks to the head and even allow them to do a complete 360 spin afterwards. In my style of TKD that is just totally unacceptable especially in self defense, and I would think these "pro" fighters would immediatly capitalize on a mistake like that. however, I feel it is because of the lack of training in countering kicks. Which is another tool in TKD that is learned.
I mentioned two kicks. The skip side kick and the back kick. Both kicks in WTF TKD use the heel as the striking surface, both of those kicks are linear kicks, if done correctoly there is no telegraph whatsoever.
Dereck while true that many MMA fighters do not come from a traditional MA background that only adds to my point of why TKD kicks are an untapped resource.
And as far as thai kicking. I see more low roundhouses done with the instep rather than the shin. I watch Muay Thai fights as often as I can and I see those bad boys using the shins, but watch a UFc even and what you see are light round kicks due to the fact that these fighters don't seem to condition their shins like thai fighters do. An example can be GSp's last match against Matt Hughes. He must have thrown 10 low round kicks to Matt thigh or shin. All of his kicks were done using his instep. Until he finally went high and caught Hughes with a skip roundhouse. Which is a kick often used, but the skip side kick is not.
Dereck good mentino on David "the Crow" L, and Ivan Salsberry. I would have mentioned them but I didn't know how to spell their names. Those guys are very impressive. They switch from stand up to to ground, perfect examples.
I am not seeing a lot of take downs being down after kicks, what I see is a lot of telegraphed kicks either being blocked or the fighters backing up. Two things that are abad idea versus the skip side and back kick. For now I'm just presenting these two kicks.
Jkogas, you are correct sir, a lot of kicks are effective against tired opponents. To reasons why I think they are rarely used to knock people out with at that point. 1. The stand up fighter is also tired and cannot waist energy on a kick. 2. The stand up fighter is not tired however has very little knowledge on kicks un order to use it to finish the fight.