Now, don't forget the Psi-Forged as well. If ever there was a perfect quori host they would be it. There's even a pretty obvious quori attack described in their fluff.
Here's my theory.
The original warforged were most likely not designed as hosts, since I don't think the quori needed hosts until after their war with the giants. Perhaps they were designed to replace the flesh and blood creatures after the war was won.
Unless of course the Quori predicted how the war would end. Which would seem unlikely at first since they failed to prevent it, but perhaps it was a unpopular theory at first followed only by a few who would no doubt later gain great staus for their prescience.
The original idea is that you take the creation forge and download a quori spirit in to a newly crafted superior and unaging physical body quickly spewing out a new army. But for whatever reason the process doesn't work, or else we'd already be seeing quori warforged armies running rampant.
Ooo, maybe the warforged were designed to be hosts, since immortal quori might be unhappy about throwing their lives away in physical combat, and it was only after the end of the war that the concept gained much greater importance and was adpated in to the Inspired.
So you have creation forges tuned to transmit souls from Dal Quor but they can no longer reach that plane. This might also explain the mindless warforged from SoX. Then house cannith takes the idea of the forge and redirects it to draw not from Dal Quor but another plane, probably Kythri the plane of chaos. It might be irian as well, but Kythri makes more sense since it's orbit is impossible to predict. A bunch of open creation forges drawing primal chaos-stuff to make souls would be fine up until Kythri becomes coterminous at which point you get the decidedly Kythri-esque day of mourning.
The fact that the warforged never sleep and thus are immune to quori possession is probably just delicious irony. The psi-forged were probably warforged retrofitted by quori agents to reverse this, making warforged that they could possess. Good thing for eberron that they've proved too cunning, and maybe rare, for that plan.
So, there that's my big theory. What do you think?