"Not exactly... the first orcs were once elves that Morgoth captured and 'perverted', to make mockery of Iluvitars creation..."Is that really true? I know Saruman says it in the movie, but I was under the impression that this was incorrect -- that orcs were created in "mockery" of Elves, just as Trolls were created in "mockery" of Ents, but that they were not actually "Elves once" as Christopher Lee ("Darth Saruman," as I like to call him) says. However, I haven't actually made it through the whole Silmarillion yet -- that text is pretty darn thick -- so it's entirely possible that I'm wrong. Did you get this from the movie (or behind-the-scenes commentary), or from Tolkien's writing? I'm all curious now.
Either way, I think it remains logical that the Orcs, whether "bred" or merely "corrupted" by the Dark Lords, need not share anything more than ugliness. However, I don't doubt that the true reasons for this in the movie are, as you say, more practical than literary.
All of this discussion reminds me, I need to get my parents to ship me The Hobbit and The Silmarillion. I forgot to pack them when I was getting my stuff ready to ship from home to college. Hey, Pooka..... :)