User blog comment:Willbachbakal/(Champion Rework) Kassadin, the Void Walker/@comment-24385282-20150301005547/@comment-1330314-20150301031944

The issue with a blink on Kassadin is that it lets him jump targets instantly and unload his burst just as quickly: while that in itself isn't bad, it becomes problematic when Kass can do that every few seconds, because it means any attempt to keep him at bay or react to him quickly becomes ineffective. Having him actually walk to his target, even when untargetable and while moving through terrain, would telegraph his intentions a lot more, and therefore allow enemies to always be able to react to his burst.

While moving through ADC autoattacks will nullify them and allow Kass to continue Riftwalking, that would also require Kass to put himself in a state where he wouldn't be autoattacking, and which the opponent could easily outplay just by moving to the side or switching targets. Riftwalk is a pure mobility and utility tool here, and on an assassin that means Kass would rather move towards his target, intercepting projectiles along the way, than just stay put.