There are a number of reasons why beam diffusers are not more widely used.
Firstly, even a high-quality diffusing material such as the POC LSD used by SureFire obviously reduces the output.
Secondly, using high quality diffusing material is expensive.
Thirdly, SureFire has several patents for their BeamFilters (ones that have a latch, and an open-stay mechanism)
Still, there is plenty of room to play for those keen to innovate.
Given the invention of the lower output, broader secondary beam by SureFire and the introduction of secondary LED(s) there is less need for a diffusing filter.
This doesn't help those wanting high output diffuse illumination though...
Al