I thought it would need some tuning because it doesn't make much sense. You bind your support for the laning phase but sometimes as the game progresses you can have better teammates to bind. You can always purchase 
but can't use it again. For a game that changes constantly throughout the course of a match it seems lazy.
It's so that it doesn't become too convenient for Kalista and her teammates. If you can change your bind anytime then it'll be too easy for your assassins, as they dont need to position themselves, just have Kalista throw you in and kill the prioritized enemy.
What if she has a long cooldown before she could rebind to someone else? It still also does that really long animation too but you can't cancel it as well so you can't do it in the middle of combat or when enemies are near.
