Weird, I uploaded it, but it wasn't in the post. It should be there now, though.

This report is accurate. For the .mtl creation the exporter use mat_wrap = node_shader_utils.PrincipledBSDFWrapper(mat) if mat else None and maps the socket inputs to the map types. Could be a design choice (?), but it's not documented in the manual.

If it is a(n undocumented) design choice, it is different from the design choice in 2.7 versions of Blender, which exported a usable .mtl . It also feels like an ill-advised design choice as it depends on a workaround outside of the program itself (manually adding the map_Kt line to the mtl in a text editor) to "remedy."

If the issue is just going to be band-aided by updating the documentation/manual, it would be helpful for users, especially the new users attracted by 2.8's more student/amateur-friendly interface, if that documentation includes information on how to export a proper MTL (or, if that's not possible in any way through Blender now, how to jerry-rig their MTL in a text editor to work properly).

It is plainly impossible to export any kind of node-based shading system into a fixed-pipeline thing like MTL or FBX formats suport. That’s just basic common sense. We can only offer some very limited approximation in some well-defined cases. Note that 2.7x did not export node-based materials at all.

@Philipp Oeser (lichtwerk) you now have commit rights.

It's strange that Blender 2.8 solved most interface annoyances when compared with other 3D packages but keep some that are still unique. I've tested most 3D packages and Blender is the only one that is unable to save a .obj file with image textures (even if this option is available in the export as "Export Materials". If we are using an image file we must add manually a mat_kd line with the proper image name after. Come on guys, this is not, by any means. a design choice, it's just clumsy and unique to blender. If I choose in the options to export materials and "copy" option available, the used textures should be copied and the respective mat_kd line added.

Took me days of pulling hair and googling to find that textures not exporting is by design. I don't think it's an unreasonable user expectation for simple shaders like Diffuse BDSF to export texture images. I can live with the workaround however. It's just extremely obscure and unintuitive.

Instead of burying it in the manual somewhere, why not indicate the limitation in the application UI. For example, "some materials in your export don't support the export of textures". Just a suggestion to help the next poor slob who has the same problem. I'm happy I have a solution finally.

As Ryan said, it's tolerable that complex textures with layers are not exported. However, image textures should be exported correctly, as in any other 3D package.

I wanted to offer a different perspective into why this should be addressed and usable .mtl files should be exported.
I'm using blender in my full colour 3D printing workflow. Because there's no repository of printable colour models, i'm having to transform animation assets into printable files.
In this context i want to have a baked on version of the texture/material and having to manually edit the .mtl seems silly given the power and versatility of blender.
Can we change this from resolved and just add the export of map_Kd "file name" to the .mtl please?