I suspect the reason only one headlight lights up is that with both of them lit up, it's possible for somebody to glance at you at night and mistake you for a car that is far away rather than a motorcycle that is actually very close. I have good eyesight, and it's tough for me to imagine making that mistake, but I have friends with much worse eyesight and it's surprising how little they see, especially at night if they have dark brown eyes.
I don't like the way it looks with just one headlight lit, though, and two headlights would improve my visibility on the bike, so I'm torn. I'm keeping the stock configuration for now. I generally try to avoid riding at night anyway. That's when all the bad accidents happen.