Well, yes, in an ideal world you're using a weapon that the enemy isn't defending and you're defending his weapon... but I think the OP's question is a bit broader than that.

As I posted in another thread thats rapidly getting spammed:

0/18/0 attack vs 0/4/0 defense (assuming max rolls) = 14 damage done
6/6/6 attack vs 0/4/0 defense (again assuming max) = 4+2+4 = 10 damage done
9/9/0 attack vs 0/4/0 defense = 7+5 = 12 damage done

So yes, with multiple attack types you get a couple more shots vs armor "weakness" but you also get rolled against 3x resulting in reduced overall damage.

For defense though, if you have multiple opponents and can't specialize, then it IS better to spread out your defenses instead of just picking one.. but only marginally.

0/24/0 attack vs 4/4/4 defense = (4+2+2 defense) = 18 damage
0/24/0 attack vs 0/12/0 defense = 12 damage
0/24/0 attack vs 12/0/0 defense = 20 damage

As for making ships that only defend, that doesn't work here. The enemy targets ships that have the highest attack to defense+hp ratio.

In other words, the enemy targets ships that have the highest attack and the lowest defense+hps first. This means that your one or two defense ships aren't going to get hit till the very end.

I've read that some people like to put one or two weapons only on a ship and pile the rest with armor. I'm sort of on the fence with this one because of the random nature of armor resistance. Doing much less damage means that you're also probably going to be in combat for many more rounds and it only takes a few unlucky hits through your armor. I usually go with a balanced att/def, generally favoring defense a little more if I have "filler" spots.

I believe I have found a sure-fire way to make me the dominator of galactic combat. Simply be the first to develop a weapon, for example Missile Weapon, well into the second generation, then develop defenses for that same weapon up to about the same place. Then sell it to every other civilization. After that, start developing your own weapon technology. The AI will go along the lines of that tech because that line is the most advanced of the three. In my last game, I successfully made the whole galaxy use Lasers, even the Drengin, and then swept them away with Missiles. Works wonders.

I have a quick question, is there a differance in fire rate of the differant weapon types? Do you get just as many attacks with missles as say with lasers?

Thanks Pierpoint for the answer. What puzzels me is how people post to use the weapon type your enemy does not defense, to me that would be fine if you only are playing one AI but each AI sets up it's attack and defense differently so if you build your fleets to do well against one AI then it could be weak against another.

What I try and do is focus on two attack and two defense lines, I then build my ships so that half have the one type of attack while the other half have the other, with both types having both defenses lines. I also tend to build the vast majority of my ships as medium hulls, they are big enough to carry lots of weapons, defense and speed while being faster and cheaper to make, I will normally have a couple of heavys in each fleet to round it out.