I would throw a lot of leg kicks in fights before my coach told me to scrap that and focus on my better weapons.
I asked him why and he said: "Unless you have the power to finish a guy with leg kicks, you should focus on other strikes that will do more damage. A leg kick can easily be checked and even if you are a heavy leg kicker, it's very hard to finish a high level opponent with it and easy to get countered".
Unless you have Barboza type leg kick power, it should be used to set-up high kicks or sweeps. Leg kicking is a long term investment in a fight, you rarely see people finishing guys with it in the first or second round. The longer it goes, the higher your chances are for a counter.
Before Sherdog crucifies me for dissing leg kicks, I'm not. It's a great weapon, but it's a weapon that rarely finishes fights. It should be used to get your opponent thinking. You hurt him a bit, get him thinking low and you start going high. When he knows you are going high, go low again..etc etc etc.