One advantage of SIWF over SIWE is that you don’t need to have 3+ different buttons / flows corresponding to different wallet providers 

Just scan a QR code, and you're authenticated

I hope that as SIWF extends to other clients, the simplicity of having a single interface is maintained

FYI SIWF under the hood is literally just SIWE only difference is that at the end of the signed message, it also contains the FID as a resource, so SIWF/SIWW is built on top of SIWE.

Plus SIWE always works on a desktop which is much faster than scanning a QR, it will take less than 2s, and just getting hold of the phone takes longer than that.

Sure you can do SIWF/SIWW with my wallet in under 2s as it also has a built-in QR scanner, in general, desktop wallets are far superior to mobile ones.

In fact, I know many people who don't even want to use a mobile wallet at all.