I doubt this would be one of those things that they actively pursue. Its likely one of those things that theyre going to say to discourage people from trying to hand off game licenses and say that if your kids want to play the same games you were playing, they have to pay for them all over again. Most people are just going to do what was mentioned before and just make sure someone has the username and password combination to access the account.
Its like Netflix saying I cant share my account with my mother. Im sure they could technically do something about it, but theres also no way for them to prove that my wife and I arent in two different houses using the same account, and since its a joint checking account that pays for Netflix, they cant exactly say that one of us shouldnt be using the account at any given time.
Im sure Valve will probably try to do something to go after a small handful of cases, like (I think it was) Apple did with people sharing music, but it would be far more costly to them in terms of legal fees and everything else - not to mention the lost business if they actually started enforcing it across the board - that I really dont think they would even do that much.
Valve sells games - thats it. If people dont buy games from Valve, they dont really get much money from elsewhere that I know of. Apple sells music, but it also sells a host of other products and services that people will still buy even though they cant share their libraries. Actually enforcing this policy would be the death of Valve, and I think they know that.
Someone's getting feisty.
There are some walks you have to take alone.