I don't think there's a compelling reason to ever get Nashor's on Kassadin.
If you're playing Kassadin as a burst assassin, you don't have time to stand around auto-attacking because you'll get sneezed on and die.
If you're playing mana tank Kassadin, your passive takes care of your attack speed pretty handily, and you need to spend that gold on survivability, desperately so.
If you're playing AS/On-hit Kassadin, you're probably dead before you entered the fight, unless you're one hell of an exceptional opportunist... and even then, you probably would have done more damage in that same period of time with an AP build. ...but I've never played AS/On-Hit Kassadin, so I could be wrong.