Such as? What would you do with them in Kansas, Nebraska or New Mexico? According to this article, there were only 12 slaves in New Mexico.
There were millions in the Southern states. It seems to me that if they were valuable in exploiting the west, they would have had more that 12 in all of New Mexico territory when it happened to look like this.
Ignoring moral and political aspects, from an economic standpoint slavery is simply indentured labor. An indentured worker could in principle do any work that a free contract worker does.