Many PCs just don't have bluetooth at all.

SFTP is out of this world for the average consumer. Have you ever seen anyone who's not a nerd use it?

My use case is quite simple: I want to transfer just any file (text, image, audio, windows executable, spreadsheet, powerpoint...) to my phone from a PC or laptop and retrieve it later from another PC. I was not prepared for the situation, so I didn't set up any application on the phone to support me. Also, I have no control over the 2 PCs in question, nor can I be sure that I can join the same network they are in.

That's the good old USB-stick use case. Only that I never carry a USB stick because all of my phones support at least USB mass storage. I don't see any other way than USB mass storage for this typical use case.