I know copying over most custom node trees from Cycles would be tricky to convert to PBR, but with the new Principled shader - most settings related to PBR are contained in one node. I'd think it'd be nice if Sketchfab could copy or translate the settings used with that node to their PBR (mettalic?) settings to help streamline the process. Would be nice to have most materials work or much closer to working directly from the .blend file, instead of having to completely re-do them each time after uploading here. Then keep in mind a lot of people aren't exactly fond of the older Blender render engine, and even the devs have mentioned it's considered a depreciated part of the software.
TL;DR : It'd just save many steps/headaches for normally working with Cycles render engine.