Right then.
It IS a legal requirement for factory fit HIDs to have both a washing system and a fast acting levelling system. That is black and white, and not open to debate.
Retro-fit HIDs is a very grey area. It is not part of the MOT, unless the examiner decides to to pick up on it for other reasons. But then we get in to the realms of just because a car can pass an MOT doesn't make the car legal. Strictly, some mods will require a VOSA test, and poorly retro fit HIDs may or may not fail this. I would imagine they would fail as unsafe.
Legalities aside, poorly fitted HIDs are probably going to be worse than standard lights with decent bulbs. The light given off by HIDs is adversely affected by even small amounts of dirt and grim (hence the legal requirement for washing systems on factory fit) - so unless you are prepared to stop every 10 - 20 miles to clean the headlights, you are going to be worse off.
Additionally, due to the intensity of HIDs, the ability to extremely dazzle oncoming drivers means the legal (when factory fitted) requirement for fast acting headlamp levelling (note self levelling suspension not fast enough) so as not to dazzle too much on uneven roads, as well as heavy rear loads. Without this levelling system, the only responsible way to use HIDs is to have them set so artificially low (as proper levelling sets them in case of system failure), that they are bloody useless anyway.