It's not a magic move and there are some real downsides:
1- If you do it wrong it can hurt like a mofo and injure you.
2- Even when you do it right it can hurt like a mofo. You're basically trying to take some pain to give the other guy more pain. Checking a hard kick HURTS.
3- If you become predictable with your leg checks your opponent will do fun stuff like fake low and go high (body or head) which has you sucking a hit while off-balance. Also, they can low kick to your OTHER leg if you're checking too high which will knock you on your ass.