He has a flash on 7/5/3 second cooldown.
Except is does damage.
Has 300 more range.
Let me re-emphasize this.
He gets the only summoner spell that is mandatory on every champion.
At worst, every 20 seconds. At best, every 3.
I don't play Kassadin, so maybe I'm missing something. But his ult as far from weak.
People really need to stop using the "b-b-but its flash on a 3 second cooldown!" as an argument against Kassadin. First off, its a fucking ultimate, it does pitiful damage so its utility should be strong. Its not like this is the only "flash replacement" spell in the game - Arcane Shift exists, and while it has a higher cooldown, it does more damage and is on a basic ability slot, meaning Ezreal has a very powerful damaging ultimate in addition to this spell. Oh, and did I forget that Arcane Shift doesn't have ramping mana costs? There are several champions with infinite dash/blink moves, its just that those have some kind of requirement whereas Kassadin doesn't because its his ultimate.
"But it does damage!" Yeah, and it does pitiful damage, even when it costs 1200 mana and you have a full mana build. And its not like Kassadin's other spells do lots of damage either. Both his nukes only have 180 base damage at max rank. And its not like his Q and E are low cooldown spells either - their cooldowns are 9 and 6 seconds respectively, and his E also makes you wait for enough spells to be cast in your vicinity. Because of his reliance on his W and Lich Bane for damage, its not like he can simply hit-and-run, he has to get into melee range of you to do damage. His damage is balanced around his ultimate, around his powerful (but nowhere close to unlimited) sticking power.
Also you should realize that Kassadin doesn't reach that golden 3 second cooldown until level 16. Kassadin is a lategame champion. He is a hypercarry. He is supposed to be ridiculously strong lategame - that's why the standard build includes Archangel's and Rod of Ages. Early game, his ultimate has a 7 second cooldown, and he barely has the mana to cast it twice. His lane phase is pitiful, and consists of last hitting with Q and W where you can.
His ultimate isn't anywhere as ridiculous as people think it is, but it should be, because the rest of his kit is terrible for it.