Feets covered it best. Generally, HID conversion bulbs do not sit in the same position within the housing as the halogens the housings were designed for. This causes light to be reflected in directions not originally intended (think of how a Maglight flashlight adjusts the beam from pinpoint to flood). Add in some cheaply made housings and a lack of any attempt to re-aim the lights afterwards and you've got a scattered mess.

I have projector housings (with HIDs) in place of the stock driving lights on my lifted truck. The beam cutoff is such that it does not go above the door handle of most cars crossing in front of me at an intersection. I run these and no headlights while in town because although my headlights are still legal height, they are annoying to other drivers when I am behind them.