Hi, good questions!
A lot of shader parametres are left over from an older era (CGI-timeline-old :)) when everything was as quick and dirty as possible. Most people are also very familiar with the options they’ve always had, sometimes with a reluctance to change. Game engines have also only very recently moved to the ‘physically based rendering’ approach, so they were still requiring artists to use the separate diffuse and spec roughnesses etc.
There is also a convenience factor too, I think. Let’s say you had to quickly create an old car tire. You may want quite a rough, oren-nayery diffuse for the dusty rubber, with then a fairly glossy spec in certain areas using a texture. You do have flexibility there.
However, layered/compounded/BRDFed shaders are definitely the way forward and even though they can require some re-learning art first, they are super flexible and more intuitive once you get to know them.
Maxwell Render only has 1 roughness parametre that does exactly as you described, going from mirror perfect reflection to lamberty diffuse if you want it. I love it, but it does mean you have to do a bit more planning with your textures for masking which areas have certain levels of roughness etc.
In regards to Arnold not going from mirror to lambert, I can’t give you any sort of authoritative answer but I know that with Maxwell they had to add a special ‘lambert mode’ switch because lambert is extremely simplified and unrealistic in how diffuse it is. So a full roughness material never quite went full-lambert. So roughness 0-99 in Maxwell is realistic roughness, then 100 is lambert mode.
Ok, all of this is just my thoughts on the matter. Make of it what you will! :keenly: