Some people like helmet mounted lights for that reason. I ended up going with a Cygolite Dualcross which is mounted on the frame, and then upgraded the LED emitters to get more light. I am probably at ~450 lumens right now, and on a dark trail it really does an excellent job of lighting things up.

The head of the light does unclip fairly easily though. If I wanted to remove it, it would only take a few seconds. But my point is that it would be a nuisance to have to remove everything from the bike that isn't bolted down. Not only the light, but the battery, the frame pump. Perhaps the cycling computer (really the size of a wristwatch). The taillight also easily unclips, so that would need to come off too. So in the end it is just easier to bring it into the building where I don't have to worry about it.

If we could just get the divide and conquer clowns calling themselves our ‘leaders’ to recognize a bicycle and its appendages as just as much of a vehicle as a car for theft punishment purposes, lots more people would be using bikes for short hop errands and commuting. Here in San Francisco, by far the biggest reason many take their car instead of their bike is out of control bike and accessory theft.

Collapsing all Mickey Mouse laws regarding what penalty one should get for stealing what vehicle into one fixed punishment for ‘theft of mode of transportation’ would promote bike use much better than all manners of bike lanes, critical mass rides, euro envious wannabe communist subsidies and whatnot. There really is no defensible reason why stealing a poor guys bike (or skateboard, for that matter) should not be punished just as harshly as stealing a rich guys jet. Except the Jet owners ability to obtain media coverage and donate to political campaigns, of course.