There's a lot of back-and-forth about the nature of Orcs, and I seem to recall hearing that even Tolkien wasn't entirely satisfied with them (If they're simply biological machines, does that mean Evil can create rather than merely twist? If they're actually twisted elves, does that mean there's a possibility of redemption, and how does that fit with nobody having any qualms about slaughtering Orcs wherever they encounter them?). Ultimately, though, I agree that an orcish ally would probably not be a great idea (unless it were a captive Objective-ally, or something).
As for Haradrim, I would like to make three points:
1. Even in the books, when Sam encounters the dead Haradrim, we get the passage of Sam wondering just how evil the guy really was and whether he wouldn't have preferred to stay at home in peace. Southrons are clearly enemies, but they're not meant to be wholly unsympathetic.
2. The armies of Haradrim encountered in the books are invading armies in Gondor, whereas this LCG cycle will take place in their own lands. We would expect that any dissenting factions would likely be found in their homeland, not in the lands being invaded.
3. This cycle takes place in a time period before the books. War has not yet broken on the scale we'll eventually see it, so dissenting factions are more likely to still be around in decent numbers. By the time we first encounter Haradrim in Ithilien, most opposition to Sauron's rule may have been silenced, but at this point it's still entirely feasible that some resist.
