If you look in the design templates folder, you can find that the human destroyer design includes:
So the human AI designs (in default setup) will always use 6 beam and 3 missile as long as they can fit to size. The focus should influence what to research and possibly also influence which beam weapon to actually equip, for instance.
It is a good idea to be aware of this if you have just one of the research/design settings automated. No matter how much you research phasers, the human AI designers won't use them. And unless you mess with tech focuses, the human research-AI won't spend much time on phasers, no matter how much you equip them. If you change research focus to phasers, I think you will see the AI research it, though.
I know what's in the templates, but read carefully what I'm posting. What you're saying is simply not true and I've proved it. I can easily get the AI to put phasers on its ship designs for humans - I just did this after I read your post, in fact.
Seriously, start a new game. Set your tech level to three. Switch around your research priorities and watch how the AI changes your ship designs. You will have to delete the previous designs to prompt it to build new ones for you, but when you do that you can easily observe how your research priorities influence the design.
There is some kind of weighting involved, though. For example, for humans the AI seems to prefer missiles to torpedoes even if you select torpedoes as the research priority. However, if your torpedo tech is a level or two higher than your missile tech, and if you select torpedoes as your research priority, then the AI will put torpedoes on your ships instead of missiles.
Phasers, beams, and guns, on the other hand, seem to be weighted about equally - again for humans, at least. I can select a research priority of phasers, and the AI will put phasers on my ships. Change the research priority to beams, I get beams. Guns and I get guns. Try it.
So it seems to be weighted according to some combination of research priority, tech level, and racial preference. (I'm assuming it's racial preference that causes the AI to put missiles on my ships even if the research priority is for torpedoes and the tech level is equal, but it could just be that the AI always does this regardless of target race - I haven't checked this yet.) The AI is a bit more sophisticated than you give it credit for and will certainly not refuse to put phasers or any other weapon on your designs "no matter what".
< Message edited by deciplex -- 4/24/2015 6:38:09 PM >