bazilisek: I know that's not what you're asking, but may I ask about your motivation? Because to reinstall from a backup, you need the Steam client anyway, so it doesn't protect you from Steam's eventual disappearance, and looking at the last fifteen years, bandwidth is becoming ever broader and cheaper, so downloading a 5 GB game off Steam five years from now will probably be akin to downloading a 500 MB game today -- not really a big deal.
I'm genuinely curious, and apologise if this seems like an attempt to hijack your thread. I have personally stopped backing up everything that I haven't made myself; I just don't see a point to it any more. The internet is a far better repository of, well, everything.
backing up your Steam games because you fear what might happen if Valve went bust is kinda silly anyway since you'd still need the Steam client and Valve's verification servers to re-install the games, unless you intend to crack the games, but then you might as well download ISOs off the 'bay...
i back up my Steam games simply so i don't have to redownload everything again if i have to (or want to) format my hard drive. at 1.5MB/s, downloading beyond 500GB of game data is not really all that cool...