The CD key thing can help a little. But often times CD keys are stolen and then implemented into a crack for the pirated versions.
And in Half-Life, cheaters that have been banned from numorous servers will try to aquire another CDkey so they can get back in.
And if an innocent person who has had their key taken tries to play while someone else is using it, they'll recieve an error message and are SOL for playing on MP.
So I'm kind of glad Raven didn't put one in, but they should have some form of protection.