For one thing, the range of kicks is very low. You have to physically be touching someone for your kick to hit them, which means that the usefulness of the kick is largely circumstantial. You can't simply kick anyone you want at any time - you have to bait them into chasing you or running through a doorway/up a ladder/ect. Even so it's easy to miss a kick due to the fact that it doesn't hit immediately and you're unable to turn or move while kicking, which, all in all, means that it's pretty difficult to land a kick even against someone that's completely clueless. It's even more difficult against someone that knows enough to circle and use footwork rather than face hugging. On top of this is the fact that the window of opportunity after a kick is very small, which means that you must have a fast weapon and swing immediately to be able to get your hit. If your weapon is too slow or your first attack fails (which happens often due to some kind of weird glitch), you miss your chance and the enemy comes out of it completely unharmed. In fact, they might even take advantage of your confusion and dismay to deliver a hit of their own.
You can manual block while kicking, but you're still immobilized and vulnerable for the duration. It's a clear trade-off, and I think it's a fair one. In fact, it's so difficult to land a kick that most players (some of them good players!) don't even try. They simply ignore that attack and play without it.
The kick is best against sheilders the same way that spears are best against horses. It's a natural counter. They have short ranged weapons, slow movement, and tend to face-hug. Of COURSE people are going to try kicking them. In fact, for players without 10 power strike and bonus damage vs shields, it's one of the few ways that you can get past a heavy shield, which you would otherwise have no chance of breaking. Hell, even from a realism standpoint it makes sense. The front kick is the classic space-making kick. If someone is crowding you, you put up a block and push them back with a quick kick. It doesn't really hurt, but it unbalances them and makes some space for you to move around in.
Basically, this is the bottom line: If you want to kick, put your shield away and go for it. Otherwise, stop whining.