I thought the same way, but any headlight over about 5 watts might overload the coil, pulling down the ignition voltage, basically making trouble.
The output of the coil will vary greatly with engine speed without a regulator of some sort.
I decided to get a bright and rechargable headlight that doesn't use the engine's output. I cut the white wire off the coil, to make sure it never gives problems.
If you do go with the power out of the engine it's best to use an LED light since they're bright on lower voltages and use less power for a given amount of light.
I'm sure someone on here will know how to set that up.
Good luck, and keep your eyes and ears open out dah.