How is Kassadin considered viable?

I bought him on a complete whim about 2 weeks ago, and I can't for the life of me understand how he's supposed to be played well. His early game has to be among the worst I've ever seen, mostly because his damage output is so low he can't effectively trade with anyone. For example, a lvl 10 kassadin with items like a tear, pickaxe, and some other small stuff has a combined total of roughly 500 damage from his whole kit (ult used once). That's laughable compared to almost every other midlaner.

His mid game is a bit better IF I rush a Ludens, but even then he's still disproportionately squishy compared to the burst it gives him. Is this champ really as bad as I think he is or am I playing him wrong?