I think it's for Adobe to implement the ability to export files in .PSD and have those extra colors in separate channels (maybe in CMYK mode plus the extra spot colors translated to extra color channels).

If it works well with PDF files why not with Photoshop files?

It's a real pain to get a 5 colors file (CMYK + Pantone color) exported to Photoshop. PSD files have all the colors flattened for the exported mode option (RGB, CMYK).

Also, it's time to think very serious about implement the Mixed Inks from Indesign into Illustrator and in the end the ability to export them correctly into Photoshop as channels.

Mixed Inks and extra channels export in Illustrator would make an even more integration between Illustrator, InDesign and Photoshop...