So, what is the purpose of Kayle 
not being able to auto during her ult from a balance perspective? Or even eventually being able to auto after reaching her final form level 16? Im genuinely curious cause I think it'd be a good idea, but I don't know very much about balancing.
Not a member of the balance team (obviously) but I would imagine it's because having an on-demand invulnerability like that isn't healthy.
The reason why it was given the delay is (probably) so you can react to it and CC her/disengage/dodge the AoE damage at the end. When Tryndamere eventually gets reworked, I feel like his ultimate will be changed significantly as well. I'll be really surprised if it isn't.
Honestly, not being able to AA during her ultimate feel terrible...it should be brought back in exchange of damage.
You can't have a ranged dps hyper carry who can on demand invuln herself and still melt your entire team before it wears off. That was original Kayle, and it was impossible to balance.
You know how Tryndamere's kit is fucking toxic cuz he can basically be invulnerable for 5 seconds while dishing out fucktons of damage?
Well they didn't want Kayle to do the same thing, since being a toxic pos to face is Tryndamere's niche.
