Honved 说:
So, thanks to Cleaving, you can stand in the back row of a formation, with a wall of shielded guys in front of you, and are suddenly killed by a blade coming through the giant stack of shields, armor, meat, and bone ahead of you?
I don't see how this is even relevant because a blocked swing won't cleave. If anybody in front of you is blocking (which they should be, they're in front) then you're at no risk of being hit through them.
Bjorn The Raider 说:
Why do I have to be worried of an enemy swinging his axe and use my shield for protection even If I am behind my soldier? Just explain me where the bloody fun is in that?
This assumes a lot of things. First, that your friend isn't going to block the incoming attack. Second, that it's going to reliably cleave. Third, that you're standing close enough for that cleave to follow through and hit you. Fourth, that you won't have time to react to this threat and will instead have to anticipate it (by holding your shield up).
There are a number of ways to protect yourself from cleaving attacks that don't depend on holding a shield up all the time. For the sake of argument, let's say that your buddy isn't going to block and that the swing will actually cleave, so what options do you have to protect yourself from this? First and foremost, your positioning relative to your teammate is the best defense you have. If you stand a little further apart from your teammate then there's no way you will be struck by a cleaving attack, and you don't have to stand so far away that you can't counter-attack. I won't explain the fundamentals of footwork and weapon reach, I'll assume you already know them, so this should be readily apparent. Secondly, if we assume you are standing too close then another option is to chamber-block the incoming attack, which is reactive rather than proactive. My point here is that you are not defenseless against cleaving attacks even if you are in the worst possible position, and you decide what position to put yourself in with your movement.
As for why we should have cleave in the first place, I would say it's to punish people who don't watch their spacing & footwork. Which is to say, it's to stop people from cramming together and encourage thoughtful movement.