armor is ESSENTIAL for fighting space creatures (because they ignore shields), and powerful armor can completely ignore minor weapon damage from ships that are a little behind technologically, and give your ships the durability they need to survive those last few shots that get through shields as you warp out.
You forgot another essential device on your ships : Damage control. Reduce damage by 40%, combined with armor damage absorbtion, ships can last a while, even against Kaltors. Still, staying out of range of space monsters is a much more effective way to kill them, the larger giant Kaltors can chew through the best armors if not quickly dispatched.
Also, shields slow you down per mass point as much as armor or anything does, and they require fuel and reactor power to function. One or two points of armor is a great insurance policy. You wouldnt believe how many of my custom ships I have seen jump out of combat for repairs with low fuel and their armor destroyed!
Hit point wise, shields offer more bang for the bucks, and recharge during combat. One of my capital ship did hold it's own against a fleet of more than 20 enemies ships for a long time due to it's massive shields; could not win, but did disable quite a few ships before warping out.