You don't have to run steam to play a valve game.

It doesn't have to authenticate to play single player.

You do have to run steam to play online, it does callup to the master server and verify.

Just like it does right now today.

It does not have to callup to the master server to play single player.

Let's try it again.

It does not have to connect to the master server in order for single player to launch.

One more time?

An internet connection is not required to play single player.

As to the "smart bandwidth", I just did a test. Steam running, downloading an update to counterstrike. Set off some pings using All-Seeing Eye. Got back my usual number of sub-100 pings. Most "dumb" downloaders will throw my pings to hell and back.