Also, wider lights are more fool-proof when it comes to aiming. I have the Cygolite Hotshot, and small amounts of play in my home-rigged rack mount cause it to wander over time, which dramatically reduced its effectiveness. The Blackburn Mars 4.0 was a much wider light and didn't have problems in that regard. It also drained batteries at an unreasonable rate, even when off, to the point that I would take my batteries out every time I was leaving it for more than a day or two.
Also, twisty roads. If cars are going to be overtaking you on a curve, you need a wider light. This is why I'm planning on supplementing my Hotshot with a floody or even uncollimated dynamo light as soon as I get the parts.