yeah, I thought about this from time to time to.
still though, portal is case in point: narbacular drop had all the portal game play mechanics - of not more - than portal. but that only made me appreciate how much the story, atmosphere and characters added to that game, to the point that the portal novelty was less than 50% for me.
regardless, valve don't seem to be doing anything 'evil' in the way that they get awesome games to us, in fact, if anything, what they are doing is great! taking great ideas from where ever they come, and helping them be the best they can be. its the type of thing that would be written on EA and Activision's Mission statement, you know... those pieces of paper left locked away in their closets somewhere, gathering dust...