AI's weaponry

Part of the reason is that they seem to trade weapon technology quite actively amongst each other, so if 1 starts researching a certain weapon type, you can almost expect the other races to soon follow suit.

I've had cases where the AI would add defenses specifically targeting my weapon type but that may only be when they are either at war with you or planning to do so soon.

Thanks. I thought about it a bit more, and I concluded I should not complain about it, weapons are equivalent, it doesn't matter which weapons AI prefers.

It's much better for you if all of them do focus on the same weapon type; that way you only have to research one defense line, and your warships are good vs all opponents, instead of having upgrade or change designs for different fronts. 

Beware though:

Some AI's will favor one type of weapon over all others no matter the circumstances.

An example would be the love affair between the Thalans and Beam Weapons.

I haven't had the Thalans as much of a threat recently.  Can you override that preference by dumping missile tech on the Thlans, or do they persist on using Lasers when they've been handed Harpoon? 

I still find it odd a bit. In my recent game AI players (all of them on Intelligent level) are fighting each other long time with nothing but ships being equiped with missile guns. None of AI players use missile defense or try another weapon type.

I think thy dummied down the Korath between Dark Avatar and Twilight of the Arnor

Turn off tech trading and they will each have to research their own paths.

I think every 50 turns or something close to that the AI are supposed to re-evaluate their ships based on their enemies ships. I have no idea when this is calculated or if it only depends upon your ship design. I know that I usually just buy their ships in most games and build constructors so I don't notice much

Although I just remembered a strange thing that happened in my game that contradict the fact the AI's always stick to one weapon only.

In my current game, everyone (with the exception of the Thalans, no surprise there) was using missiles, including me. Then, during my war with a custom race, the Terrans surprised me by showing that they have developed beam weapons while I was not looking.

Strange, don't you think?

Oh, I've seen the AI switch weapons occasionally. One DA game had everyone using either missiles or beams, and using defenses against them as a result - which gave the Krynn a big advantage when they suddenly started using mass drivers, even though it set them back. In the TOA game I'm writing an AAR for, both the Korx and the Altarians had low-tech mass drivers, but when I traded them Laser V, they switched to that and subsequent beam weapons - and then the Altarians started going back to mass drivers when their enemies' shield technology got too good.

So it does happen. I've never seen the AI using multiple weapons at the same time, but I don't do that much myself, so I can't say there's anything wrong with that.