Unmodded game, pure kinetic is significantly worse than mixed arms. AI is better with flux management - and less reliant on shields - in 0.8.x than before. I tried pure kinetic Hammerhead against enemy Dominator, and almost lost due to CR decay and time out. Then, I tried 50/50 Hammerhead against the same Dominator and the fight went much faster. Armor is already a significant defense, sometimes.
Also, there is that one combat skill that halves kinetic damage on armor, effectively turning kinetics into fragmentation damage against a ship with that skill.
P.S. Combined with AI cowardice in fleet action, it is important to hit for big lasting damage when the shields drop and you get an opening before the vulnerable ship withdraws and another fresh ship covers it and takes its place.