not only for hair.

i will give an example. in reality lights are brighter then 1. thats why we use HDRI images. but we can fake it in poser. lets say that i have a big box in my scene that represents the light source. it will be reflected on my reflected materials. but in reality the box would be brighter then 1.  so i would set the ambient to  3 or 4. now the reflections are better. now comes the problem. when i use IDL it will look up the white box and it will add extra light. if i have ambient set to 4 then IDL will give us even more extra light. 
its IMO a problem.

so if it is possible it would be great if we could turn of IDL on specific materials.