They don't need it like Microsoft did. Microsoft as a gaming company was an unknown quantity and their debut was back when games were "kids" activities so they had to market heavily. Valve on the other hand is a well known game company with a proven track record, even in the console space. Try and find a console gamer who hasn't at least played Left for Dead, Portal, Team Fortress 2 or Half Life 2 on a console at least once.
Also, the age demographic has shifted since the introduction of the Xbox more than a decade ago and many of the people who are going to be getting the next gen consoles are are adults who have been playing games for years and again are well familiar with Valve and probably Steam as well. The big push isn't going to be to market to the soccer moms, it's going to be to appeal to the discerning gamer.