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.
But going from nearly perfectly smooth lines to perfectly smooth lines should not be a profitable market. Nobody will notice the difference during any real gameplay. You can already do great anti aliasing so why would someone pay $1000+ more for a 10% or less gain.
I think the main purpose it serves for Nvidia is branding. This gets plastered all over the tech news and everyone knows Nvidia makes the fastest GPU. People then associate them with better technology (for better or worse).
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.