Kishin said:
Warforged sentience wasn't the original intent of the experiments...
Yes and no. Aaren d'Cannith was trying to create a new form of life. However, the key factor is that Aaren was trying to create LIFE - not the perfect soldier. He, in fact, opposed the use of the warforged in battle and was excoriated from House Cannith as a result.
So, the HOUSE didn't seek full emotional sentience. But Aaren did, and Merrix simply seized and took advantage of what his son had done. And yes, as others have said, even Aaren's breakthrough was largely a matter of adapting the old magic of Xen'drik - rediscovering something instead of creating it from scratch.
Aside from the question of whether you can actually buy an iron golem, the key is that a squad of intelligent warforged soldiers can perform many actions that neither humans or iron golem can replicate. If they possess Craft (which as intelligent beings they can), warforged can repair themselves... or maintain the armor or weapons of their human comrades, if there are no warforged to repair. Needing neither sleep or food, warforged make excellent scouts or deep strike teams, moving into territory controlled by the enemy where no supply chain can be maintained. And again, with no need for sleep, they make great guards, manning the walls of a fortress at all hours. And as scouts or guards, the fact that they CAN respond in a creative manner to unexpected situations is the key. Likewise, the point made earlier about holding a battlefield: once you've captured the enemy city, which is more useful, a tank, or a hundred soldiers that never need to sleep and can be repaired if they are ambushed by insurgent attacks?
Tied to this is the fact that warforged possess class levels (as I said, I see the typical warforged as a 2nd-level character - I've discussed elsewhere what this could mean for your 1st-level warforged). Which means that a warforged can be a fighter, but he could also be a mithral body scout, ranger, or rogue... again, capable of recon and intelligence gathering operations beyond the capabilities of the golem. And as others have said, the warforged can even improve his abilities over time!
For a frontal attack, the warforged is inferior to the golem, or for that matter to the warforged titan. But they key to the warforged is that they are tireless and self-sustaining, and yet capable of taking orders and responding in a creative manner to problems they encounter... something the golem cannot do. They serve a different role than the golem, and in that role - especially for raids or scouting missions behind enemy lines - intelligence is certainly an asset. Emotion? Not so much, and that's something Cannith tried to surpress as best as they could. But that was Aaren's work, and it was part and parcel of the Xen'drik-inspired breakthrough... which comes to the role of the warforged in history and the world, not just in this war. Who created the very first warforged, and why?