Personally I find having a higher frame rate in faster paced game is far more important than having a diagonal line be perfectly smooth. In the middle of a game you'd never be concerned with how perfect the image looks.
Why someone would buy it? There are a lot of people who want the very best. It's the same reason people buy an Aston Martin over a Toyota.
From a more practical perspective though, there's also future proofing. This card will be able to deliver high frame rates for a few generations of games. People who don't want to regularly rebuild their rigs might prefer this card.