Why does Duskblade exist?

AP midlaners are already in a bad spot at the moment, so why does this item exist? It's as if Riot wants to make AD assassins as simple to play as possible. Speaking of AD assassins, who's ready for to reach necessary-ban levels like ?

I mean, really. It's hard enough to lane against a manaless champion, but now you have to make champs like Zed easier to play by giving them an item that essentially makes it so your chances of surviving a level 6 towerdive are 0%? Just because a champ reaches his ult doesn't mean he should be able to completely ignore the peashooter that is the turret. Don't even get me started on the Dragon changes...

So again I ask, why does Riot torture us so?