Considering how much trouble Steam has with phishing and accounts getting hacked, it's obviously a good idea to minimize any use-case where users are allowing others to get at their passwords. This kind of feature will probably save Valve money in the long-run.
Still, doesn't match what I hoped. I was hoping to see a way to share a few games on a machine within the same house, so I could have my set-top machine and my desktop machine divvy up the console-style and pc-style games without constantly having to re-log-in.