Let me explain this as quickly and easily as possible, since I know the topic well. You entire headlight is designed to have a filiment on a bulb in a very specific location. If it is even .1mm off, you beam can be off by 10 feet down the road, or you can loose up to 50% of you usable light.
Now, an HID capsule has no filiment, it has a bubble with halides and salts in it. This bubble is about 8 times the volume/reality of the filiment it is replacing. If the light is designed for halogen, and you used an HID capsule in its place, you are completely ruining the optics of the light. It might appear brigher looking at it, and might appear brighter against your garage door, but 200 ft down the road you have about half the output of your halogen bulb.
Now, headlights designed to be HID are a whole different story. Also, those re-based capsules are running 25K volts through an unshielded cable. This is a fire hazard and an RF nightmare.
