The arasaka 3000 is really good, but thats pretty much the only thing I use cult arms dealers for. I think its fine the way it is though, allows access to decent weapons without promotion II. I'll never get use out of it, but if you're doing a first time playthough and don't know to get promotion II quickly it's pretty helpful.
FWIW, I have never used Arasaka 3000. It's kinda sniper-like rifle, with almost as good armor penetration, yet it does not depend on firing accuracy for accuracy or damage, but gives damage boost for reactions. I guess it has some niche use outside when your firing accuracy isn't good enough (80+, especially with .308 CAL) to use sniper rifles. SVD is somewhat similar in this respect; I never use it myself either.
IMHO it's just so much easier to use other means to train firing accuracy up high enough to be able to use sniper rifles. But I agree that these could have some use until you do.
As for Cult Operations. If you want to optimize the game, you usually don't want to research any Cult Operations until you can do all of them. This is because researching Operations stops Safe House spawns, which are rather easy missions and can provide a lot of loot (even up to 1 million cash). You want to churn those for XP and cash until you can get to the next phase of the game. This requires all cult operations so that you can get OSPREY. Without osprey, you have no chance at dealing with Cult Forward Bases, so why not keep milking safe houses and outposts until you can deal with them.
