well, let's put it this way. the focal point of a halogen lamp is a skinny filament. it is ~5mm long and less than 1mm wide. The factory housing puts low levels of light up close, and increases the intensity as the beam goes farther from the car, putting the hotspot up on the horizon and then slaps in a sharp cutoff above that. It is extremely precise. And it throws most of the light forward, not near the bumper, so you have better distance vision, and so YOU have better distance vision WHILE keeping glare out of other folks' eyes.
HID capsules are a 3-4mm sphere. and the whole sphere glows. but there's also a hotspot in the sphere, an arc shaped like a rainbow. It's a HUGE lit source compared to a tight filament. The reflectors just cant control it. Try wiggling your oem bulb around and see how the beam distorts up against a wall. Imagine getting ALL of that distortion at once. That's what HID does in a halogen housing.
What you gain: lumen output. and your friends that don't know any better think it's cool.
What you lose: controlled focus of your light. you may perceive greater vision, but you actually risk losing some due to too much light up close.
What they lose: they get glare in their eyes.
Unless you try to lower the focus, to reduce the glare, the responsible thing to do. But now you've increased the light in your foreground, and hurt your distance vision even more.
There's a lot more that goes into engineering a great lamp than you'd think. Stock, the WK has excellent lamps and mediocre brights. But the lows are hard to beat. Slapping a chinese HID kit in them is counter-productive.
Projectors CAN do a much better job of controlling the beam pattern, if they are set up properly. As a retrofit, it's the only way to go.