OK, I'm not so good at math, so bear with me.
OK, so "off type" defenses are reduced to the square root of their value. So if you have shields valued at 4, and you get hit by a mass driver, your defense is 2. OK.
So, this means that all defenses are equivalent at value = 1. Since the square root of one is one, there is no difference between any of the defenses at value 1.
Now, when you make a defense roll, you roll between one and value of the defense. So, if you've got a defense of 4, you roll between 1 and 4. You do this for each kind of defense.
So here's my question. Isn't it better to have one of each defense, rated at 1, then it is to have a defense of 3 for one of them, even if you know what you're facing? Say you're fighting a guy who only uses beams. Aren't you better off building ships with 1/1/1 defenses, rather than 3 shields?
Because the average roll of defense 3 is 2, and the defense of a 1/1/1 ship will ALWAYS be three.
Maybe I'm overlooking something.