There is a fair amount of hum that isn't there in any other position.
I think someone mentioned somewhere that if they had wired it X there wouldn't be any hum.
The flaw with the OMG mode is that both pickups use the bridge-side coil as the unaffected coil, so it works like two single coils that are would the same direction and polarity - i.e. it doubles up on the hum when you use both together.
If you're feeling adventurous and are good at wiring, you can mod it to reverse the affected/unaffected coil in just one of the pickups, and you would get hum-free operation when both coils are on (like a Jazz bass).