Strongly disagree with the notion that head-mounted lights are "inconsiderate." You can easily glance off-center to direct the main beam away from the oncoming pedestrian or car. Car headlights are WAYYY worse than bike lights unless you specifically point the bike light directly into the eyes.

From my direct experience, head-mount lights are extremely useful, and ALWAYS improve your view of the road, whether you be on or off road. Even if you just use a "to-be-seen" head-mount, the ability to shine it in a certain direction to get the attention of cars who don't see you is invaluable. The notion that a head-mount light makes for inferior visualization on the road is not true at all, shadows or no shadows.

If I had a choice of single mount light, I'd choose the head over the bars. It doesn't matter if you've got 10,000 lumens - if those lumens are pointing in the wrong place, it'll do you no good and also increase blinding of oncoming peds/cars. 200 lumens (one Fenix flashlight) pointed in the CORRECT location will suffice for the VAST majority of realistic night riding situations up to 20mph, and even off-road.