First off, most MMA fighters come from predominantly wrestling and/or grappling backgrounds - they have limited striking experience. So they are not going to go for a head kick in a MMA fight. A good grappler or wrestler will take a guy throwing a head kick with ease so if you are not a good kicker it is unwise to throw a head kick in a MMA fight. How ever I've seen a few head head kicks thrown in MMA. Guys like Frank Shamrock, Chuck Liddell, Guy Mezger, Yuki Kondo have all thrown head kicks. I've even seen Tito Ortiz throw a head kick when he fought Randy Coture.