This seems rather inferior to Lightning. Aside from getting rid of excess Fractals or another unneeded spell, I don't see much use to it...
That's the point.
Arcane Blast is meant to be a desperation Lightning, or a "win more" kind of card. It's not meant to be used every turn. Lightning on a stick (reusable 5 damage to any creature or player) is a ridiculously powerful effect that's easily worth a ~6
ability cost when it comes on the Mark. The heavy discard is there to prevent it from being used frivolously.
I thought about changing the discard requirement to creatures, but Fractal Is Why We Can't Have Nice Thingsā¢. Permanents as the discard requirement lacked flavour, but discarding spells to fuel the ability made sense.