This post reminds me of that tournament match (forgot exactly which one) where a Kobu Jutsu Tanya was brought down to 5% health or something, and the Noxious Reptile had about 1/3 of a lifebar, and the Tanya player ended up winning through relentless chip damage - the poison damage didn't even come close to saving them.
As for who needs what, Sonya definitely needs her dive kick, especially in Covert Ops, because without it her approach options would be pretty limited, and she can't zone to save her life (well, maybe in Special Forces...)
And as frustrating as it is, Mileena critically needs EX Roll, because frankly, if you're going to have moves that unsafe, having some sort of overhead threat is pretty important...