In the case of Skyrim, the game didn't work AT ALL with Crossfire on my system when it was first released. Had to disable Crossfire at first, then ATI released a profile that finally got it working, but it wasn't a performance boost, it was a bug fix.
I'm not saying Crossfire/SLI didn't give any performance boosts, what I'm saying is that for most models of graphics cards, it is a marketing gimmick to get you to pay twice for something that might average out to a 10% performance boost across all your games. In some cases, it was buggy and causes games to crash, in some other cases, the games didn't even support it.
Yes, at the bleeding edge, with super high resolution, you probably require SLI or Crossfire, but for those of us not in that 1%, who build our PC's within a budget, SLI and Crossfire are a waste of that budget
Bear in mind, this is just my experience. It's purely anecdotal, take it with a grain of salt.
It's possible that newer cards may have gotten a lot more efficient at SLI/Crossfire, but given my experience last time with video cards, I can't justify paying for something twice when I can't see any real noticeable improvement.