This is the current (july 2018) GLTF status.
Test case: the same skinned mesh exported from 3DS Max 2018.
Standard material: Diffuse + Specular + Normal
SEA3D exporter + SEA3D importer:
SEA file size: 658 KB
Result: close to perfect
Babylon3D GLTF exporter + GLTF importer:
GLB file size: 1,850 KB
Result: messed up materials
I’ve also tested the FBX2GLTF utility (by Facebook): the same wrong results
Important note: there is nothing wrong with THREE.js and PBR materials:
In any case, PBR was a bad choice for GLTF and also, all the current converters are collection of bugs.