Some comments on several good points made here regarding head kicks:
Chance of slipping on head kicks - I have seen many fall on their butts doing high kicks on a sweaty, slippery dojo floor (myself included) but have never seen someone (adults)slip on a low or even mid level kick. High kicks are inherently less stable than most other techniques.
Head kicks work in MMA, sometimes spectacularly - true, but not that often and only by proficient kickers. Important to note that groin kicks are not allowed. If they were, I bet there would be a lot less head kicking. They also work in karate competitions, but that is also facilitated by the rules which encourage longer range attacks; close in elbows, joint attacks, knees and forearms not being awarded points in general, so the engagement distance is typically further than in very CQC or old Okinawan karate.
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The opponent's style has something to do with it, too. Some aggressive close in guys will take great advantage of an attempted high kick, such as mentioned above. Throwing a head kick that misses with the opponent in good position to counter is very bad, especially if the situation allows for groin attacks, in which case its very, very bad.
Having normal hip joints, I can't head kick a guy 6 foot, 4 inches tall even when I was 20 years old. Someone 5 foot, 4 inches, I'll probably go for it if its the right move, but most of the time I can use a mid kick to set up a head punch just as easily, and safer. Might fake a head kick, though, just to keep the opponent guessing.
So, based on the above, I think that in a few situations, a high kick or two may be OK, but in general, across the board in various scenarios, they do not seem like the best option to me.