Were I to make "unauthorized backups" of my games, Steam would shut down my account completely, by means of the "we can do what we want for no given reason" clause of the ToS.

I'm not even speculating, I've seem them do this - a guy owned L4D2 and complained to them that he had stability issues with singleplayer and had to play a cracked version while they got around to fix the network code. His entire account was banned for "piracy".

So fair use shmair use, I guess. You can still be punished by service termination and DRM lockouts even if they can't actually prosecute you.