PBR lighting; Transmissiveness; 3D model improvements & more.
Phong model is primitive comparing to PBR, but it's very performant even on potato gpu.
As a hand-paint artist who works mostly with diffuse textures I can benefit greatly from having Phong along with PBR.
On other hand, if Construct 3 in general benefits from single model architecture - then I'm all in for PBR only. Especially if it helps in speeding up GPU skinning implementation.
For a mostly diffuse texture, doesn't PBR with 0% metallic and 100% roughness basically look the same?
True, but sometimes I can get away with the very basic shading that Phong can provide and it will still look good with the right diffuse texture.
Meanwhile, usually, PBR still calculates all of the needed data, even if it's set to values that give visually same results.
My performance concerns are not critical, it's more like I can imagine some cases where I can win some performance by choosing Phong over PBR, when I try to push for more objects.
My main reason aside, I feel that old-school 3d graphics are entering their "pixel art" period now. I mean people are trying to achieve psX, ps2, old PC looks. For such tasks having Phong would be pretty cool and very authentic. It kinda has it's own vibe and looks slightly different to PBR.