Another reason I hate Kalista
As a support, Kalista's oath forces you to spend all game with her, and after the laning phase I try to avoid the ADC unless its for teamfights, or pulling their slightly singed behinds out of the fire. The support role is more than just babysitting the ADC, its about helping the rest of the team with ganks, wards, and objectives.
See, I'm warding crucial points on the map and helping allies win what is left of their lanes either with a well-timed exhaust or assistance in pushing the lane. Also sometimes I'll grab the ruin potion (really good one imo) and go split push a tower if we're not going to teamfight. I personally hate the ADCs but I'd rather they not feed like crazy, which is why I support them so they don't die.
Kalista requires a support to babysit all game in order for her passive % damage and ult to work, which if she is farming and not helping with map awareness or dominance (yeah a ton of ADCs play with tunnel vision) then the support has to be there, severely limiting the usefulness the support has.
I think her oath should be recastable on another ally for this reason or maybe make her % damage work at 1/2 strength without an Oath; as it stands having Kalista on your team kind of ruins the supports global effectiveness.