If you play as an Orc / Uruk Hai, would you be able to fight for one of the good factions (Rohan, Gondor, etc)?
- Theoden, have you got quest for me?
- Ahh... Hello Ugruk, yes I have. Could you commit suicide you ugly, nasty orc!
Haha, right on. If you look like an orc, you can obviously belong only to factions that have orcs, and they are all evil, there's no switching sides or flexible feudal allegiance like in MB - you choose your home faction during character creation. Of course, you can go over and help out the human Umbar pirates, Khand, Rhun, Dunland or Haradrim, and recruit their soldiers, but never change a faction.
The only exception to this if your faction is destroyed, then you get to choose a faction from your surviving allies, and become one of them.