If we say, built Ithilmar and VGA and sent heavy and super-heavy infantry alongside our allies, we'd be contributing.
We're probably much more capable of deploying those heavily armoured troops without just getting them killed as well, and as they add a capability our allies don't already have, they'd probably be much more grateful than us sending something they can already do better (as any green line troops we send with Brigandine and Needlers would be significantly less useful than their own experienced troops with Light Semi-Powered Armour, Hellguns, Flare shields, and grenades).
As I say, the question isn't Brigandine or nothing, it's why Brigandine rather than any of our other wargear.
Yes but then we would be throttling how many soldiers we can field by how many Ithilmar suits we are making.
We picked Brigandine for a variety of reasons, all of which have been stated by various people. But I shall reiterate.
Wraithweave-and-Needlers are a test case, our first foundry so that we can gather data on what our foundaries do (all our tables are probably screwy post-fall) and how much of it they do. Its so we arent spending all of our bonesinger AP making stuff all the time. We build a foundary and just works forever spitting our armor and guns. Because of how the costs of the armor and the gun shake out, its a nice, cheap, simple test case so that we can plan our future foundaries.
We need *SOME* suit of armor now, and wraithweave are cheap so the hope is our shiny new foundry will produce a bunch of the things which will last basically forever until destroyed or lost.
Our idea is to be the arms dealer of the Aedlari, and so giving them better suits that suit their tastes will just be good sense both from a trade standpoint and a 'We dont want the Aeldari to become more of an endangered species than we already are' sense. Wraithweave Brigandine just seems like it would fit with what we know of the Asuryani.
These foundaries will be all about ratios. If we want to automate the raising of detachments we will want foundaries to support those detachments. Because they need kit when they finish Space-Elf-Bootcamp. So having a foundary that just spits out stuff as fast as we can train people just seems like good sense.
Having extra for an emergency is also good sense, and these seem like decent kit to hand out if say, Vulkan decides to pay us a house visit and tell us what he REALLY thinks of the Aeldari. We can throw civilian militia in these suits and shove these needlers into their hands.
So there is a variety of reasons why we picked this suit and this gun.