Why does Kassadin's E still require charges?

I've never understood this even before his massive nerfs. Now, it just makes absolutely no sense. His E is gated by a cooldown and still requires mana so what's the point in this charge system? He can't poke with it and most likely wouldn't be able to use it more than once in a trade, with or without the charge system. There are other slows in the game with higher AP ratios and without this whacky system. Can I either get an explanation to this or get support to remove it? Maybe it made sense when he was good but now he has tons of room for buffs, and this just seems like a good first thing to look at.
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