I find the current interface to be a bit confusing. When “Use Nodes” is off, Viewport Display is a misnomer since changing the Viewport Display color and roughness will affect the render. And some settings are available in both Surface and Viewport Display. Roughness and Specular color don’t even affect the viewport right now but I’m guessing that’s just a bug. The only “viewport only” setting seems to be specular color.
I thought maybe the Surface sections could become a toggled section (eg. like Gravity), with node settings greyed out when toggled off. Viewport Display could become Default Surface or something like that, with all the settings for the non-node material.