It's an excellent strategy. To add to your examples: Team Fortress was originally a Quake mod, and Valve hired its creators to head Team Fortress 2's development. Counter-Strike was originally a Half-Life mod before Valve bought the rights and made CS:S, then CS:GO.
I get the point, and it is true to an extent that the best products/games are ones that people didn't even know they wanted (the original ipod being a good example), but this faith in valve is misplaced at this point. half life was an incredible achievement, but it was released over twenty years ago. the last great original title (imo) created by valve was left 4 dead, released over a decade ago. I definitely like steam better than any of the other distribution platforms, but in terms of actual games, they haven't done much worth talking about in the last ten years; they're just playing out the endgame on their old IP.