Hmmm...I thought that "balancing the force" meant a balance between light and dark sides in one being (or the elimination of the light and dark sides altogether and the creation of a new jedi order). However, I'm not a huge fan of Star Wars (I've seen the movies, collected a few of the Dark Horse comics, and still have all my old action figures), so I may be wrong.
Actually, saying that Anakin would balance the force and implying that it would be a good thing, only to have it mean killing a whole bunch of people seems way too ironic for Lucas (I don't think that Lucas really understands irony).
True, when he killed Obi Wan while Luke began training returned the balance to 2 (Palpatine & Vader and Yoda & Luke), he totally screwed up that balance when he killed the Emperor and himself. In the end, he left just Luke.
Granted, you could argue that Vader and Luke were the only ones left initially, but Vader had turned by that point, which possibly means 2 light (3 if you count Leia) and 0 dark.
But, like I said, I'm not really knowledgable on the subject of Star Wars, so I might be arguing against official "cannon" (although Lucas seems to change that at the drop of a hat).