I've recently supported Kalista for the first time since she came out. Mostly my experience with her is playing AS her. I didn't really realize how disorienting it can be when you are ulted by her when you're playing with a random. Before, my oathsworn would comment on it but more in the wow-that-was-weird way when she first came out, and I admit I mostly used her ult defesively or waited for the command from my partner whether or not they wanted me to ult by ping indication.
Now I understand.
The problem is not with not being able whether or not to accept/deny being oathsworn, THAT you should treat as any other variable of the game that you have no control over, it's that there's no safeguard for misfiring the second activation of her ult.
The fact that it can be reactivated by right-clicking is a terrible design choice. I can instantly tell whether or not other players are notorious right-clickers like myself based on the time in-between being ulted, it's second activation, and where they go.
If it could be reactivated by ability use only I can guarantee that it would be less toxic. In fact, it's one of those innocently toxic abilties. You can use it intentionally to be a jerk at bad moments, but with more control over it you can reassess and take advantage of it rather than being this great disorienting thing that JUST HAPPENS TO YOU when you're playing with a Kalista you can't communicate with real-time, or at all.
It's difficult to come up with a good analogy for this because of the variables, but it feels like the difference between seeing a car come at you at 70mph vs 30mph.
Maybe if there was a way for the Oathsworn to queue up to be ulted. That'd be nice. But the state that the game is in now the mechanics don't really allow for it. It'd require one of a few things.
- Taking an item slot up for the Oathsworn (which sucks)
- Adding a conditional 6th ability slot (seems messy)
- Adding a type of utility belt (which would open up loads of options that would be highly beneficial to supports and junglers especially)
- Adding a special key assignment for Oathsworn
The current reality of it though, is just that she's a conditional pick. It's not a good idea to pick or play with Kalista when you are not partied with someone you know/can rely on.
It's like picking Katarina against Viktor. Occasionally you'll get people who can play Katarina and not just survive, but thrive in lane against him, but most of the time it is really just a bad decision.
I really can't wait for teambuilder to become the norm because that's where you really have the most control over who/what you're playing with.
getting pulled out of their ults, 
getting ulted right before a dunk so the Kali player can send an auto and get the kill instead, 
getting flamed for not sending her squishy, goat body toward the opponents. There was even a whole thread about a support 
who absolutely refused to be turned into a projectile.
