It seemed like the only function of the capitol, late game, was to build a unit to escort a purchased settler to an iron deposit.
Yes, but if having enough happiness it's not a bad idea to annex a recently conquered capital and rush buy units, especially if you think you will not have sufficient forces to take down last civs.
Does Persia is also strong because he can kill everyone without iron? Great synergy with going honor for GG. Rush buying immortal from annexed cities can be a good idea.