OK, so here's the way this goes.
1. We know that Harry's scar was inflicted during Voldemort's attack on him when he was a baby and that the scar gives him a connection to LV.
2. Part of this Harry-Voldemort connection involves a transfer of at least some of Voldemort's powers over to Harry. I'm thinking in particular of the fact that Harry is a Parselmouth.
3. What if the scar gave Harry more powers than just the ability to speak to snakes? What if Harry is actually a Squib, and the only reason why he's a wizard is because his scar gives him some of Voldemort's magical abilities? [Note that powerful wizard parents don't necessarily have magically gifted kids. Observe Neville's almost-Squibness in books 1-4, despite the fact that his parents were powerful Aurors.]
4. There's been speculation flying around since HBP came out that Harry's scar is a one of Voldemort's Horcruxes. As said above, certainly the scar connects Harry to Voldemort, and being a Horcrux would certainly explain this connection. Let's suppose that the scar is in fact a Horcrux.
5. Assuming that the scar/Horcrux can be destroyed without killing Harry, then afterwards, Harry would revert to being a Squib. In other words, he would only be able to defeat Voldemort by giving up his magic and his active involvement in the Wizarding World. This would be a far more devastating sacrifice than if he were to have to die.
So that's my half-assed pseudo-theory -- that Harry killing Voldemort might require him giving up his magic. I'm not saying that I think this is going to happen. I don't even know if this is consistent with canon, since I haven't really done much digging below the surface to find out. But I'd love to hear what any of you have to say about it.