Front kicks (teeps) to the thighs and hips are some of my favorite attacks. The main problem I see with them is that if you aim for a thigh and get shin checked into the ball of your foot, it can hurt like a bitch. It has happened to me more than once, and has left me limping for a day after. If you use your heel as a contact point it's nowhere near as bad, but you also lose a decent amount of reach.
Also front kicks don't pack much power when compared to a rear leg roundhouse. A front thrust kick can do some good damage, but it's tricky to find the right distance to use a kick like that effectively. Although a roundhouse always has an ideal striking surface, I care much less if someone step into my round and gets hit higher up on my shin than if someone stuffs a thrust kick and I lose my balance.
I'm a bit scatterbrained in my organization of this one, but I'll sum it up and say that a teep to the thigh is a great kick for keeping distance or setting up your hands, but that front kicks don't deal much damage compared to a solid round.