The thing with a cutoff light beam is that you get the light where you want it. On the road in front of you so you can see the road without blinding traffic. With, let's say BT21, pointing down on low you will see much less of the road and still blind traffic more than for example a Philips Saferide (I know I have both lights and I have done som tests).
While I can see some advantage to a cut-off beam pattern for commuter paved paths where there is the possibility of direct on coming traffic, I don't think that such a design is necessarily the best option for road use. On the road you actually want to be seen by traffic that is approaching and you also want to see things in the peripheral that might become a hazard to you cycling at night. From that perspective a "cut-off" beam pattern could be considered counter-productive. Unless we start talking about lamps that emit over 2000 lumen and are in very close proximity to another person I don't think any lamps designed for bike use are going to be a major hazard or annoyance to vehicles that are driving down the road. ( when used properly )
If you routinely drive at night ( like me ) you see many sources of bright lights that are terribly annoying. There are cars with the new LED/HID lamps, trucks with lamps that are mounted higher, emergency vehicles with high power strobes, road construction crews with lamps designed to mimic daytime conditions, the list is almost endless. Not to mention that almost everyone has had to drive into the sun during sunset at some point. A bright bike lamp pales in comparison to ANY of these things.
If you're going to criticize a cyclist using a bright bike lamp you need to put all other road factors into the proper perspective in order to give the night cyclist a fair shake. It continues to baffle me that I consistently read ( from other people's posts ) how very bright bike lamps are, "Blinding to road traffic".
To me this idea is one of the biggest urban myths I have ever come across. When I drive at night I get blinded almost every night by various vehicles or various sources of bright light. When I say "blinded", I'm talking about the need to either look away from where I'm driving or to adjust my mirrors or to lift my hand to shield my eyes. The only time I've every been blinded by a bike lamp was while mountain biking on trails and I came upon someone else using bright lights ( direct head on traffic that passed me in very close proximity.)
Lately I've been seeing more people using bright LED bike lights when riding their bikes on the road. This pleases me because the last thing I want to do is run into someone riding a bike at night. The more these people stand out the better and more safer they are going to be. Never, and I mean NEVER have I ever had to either turn my head away or lift my hand to shield my eyes from someone riding a bike with a light that was too bright ( when on a road ). In the future if that ever happens I'll be sure to talk about it.