I would say that there isn't really 'a point' in having 2 Guard units, rather they're both meant for specific roles.
You said it yourself, the Iron Guard protect traders and would probably form a specific group, perhaps paid buy the King in that area, or perhaps just hired by merchants to look after their stock on the more dangerous routes.
The Khazad Guard, however, are part of the standing army of Khazad Dum, and fill the more conventional 'guard' role of protecting force leaders and royalty.
I personally think the Iron Guard fit Middle Earth quite nicely, as they have their own niche, but I can see why others would disagree, as it may only be in desperate times (e.g. when all efforts are devoted to war so no trading is occuring) that they would be diverted to serve in the main army a long with the Khazad Guard. All depends on the theme of your army really.
Hope that helps.