The rule became necessary due to people putting any bulbs in any lamp units and causing dazzle to other drivers.
It's not as simple as stuffing anything in there that will light up.
Lamps, lamp units and lenses are a very complicated science. All those lines you see on a halogen lamp unit glass are not there just for decoration, it takes into account reflection, refraction, defraction, beam pattern and other stuff. Millions of pounds have been spent in R & D to create an end product that works as it was scientifically designed to and then somebody decides, "Nah, I want to put these in so I will" without thought or consideration as to what would happen.
The proliferation of badly-matched lights and the dazzle caused by them were the driving force for this MOT regulation to be created.