Most MMA fighters don't know how to high kick. They train for leg kicks and maybe a front push kick. I was watching a show called Fight Quest, where a couple of MMA fighters (one pro, one amateur) went to various countries and learned a martial art from that country. The pro couldn't kick above the waist level with any power, because his leg flexibility was so horrid. Look up on youtube "Top 10 MMA Kick KOs" or something similar and you'll see some pretty good ones. There's even one where the kicker jumps off the fence post for added momentum. From what I've seen on various MA-related shows on Discovery, you can get about 140% MORE power into a kick than you can into a punch (a boxer's cross rated at 1000 pounds of force, vs. a roundhouse kick at 2400 pounds). When people connect with a punch, sometimes they knock the other guy senseless, and others the other guy just takes it. Very rarely do you see a head kick connect and the other fighter doesn't go down.
Left leg forward doesn't mean you only kick with the right leg. You can do skipping kicks or sliding kicks, plus a good TKDer should be able to operate from left leg forward or left leg back (as after you do a kick with the right leg, you're quite often right leg forward).
I'm not saying "go kicks or go home". I'm saying that if you can land head kicks, it's a very powerful tool to have in your toolbox, that if you have the opportunity to use it then it could be what wins you the fight.