why the hell would most people want to spend an extra 300 bucks or so to have an extra video card at only 25% or less extra benefit in framerate?
SLI will give you better than 25% improved performance. I hear the cards can double performance. Still most games will stick to one video card. The Valve hardware survey has SLI users at 2% of valve customers. That's 2% of gamers not everyday PC users.
You don't need to spend 300 bucks on any video card. Most 100 dollar video card give you all the performance most people need. People are even playing video games on the Intel 965 embedded chipsets. Usually you see people using SLI if they've already pushed the upper limit on video cards. These are the people spending over 300 on each card and can't increase performance with a better card. They've hit the ceiling and SLI does deliver the performance.