Non mono-spaced fonts for programming... An interesting idea but with one caveat I can think of. Many programmers, although not me personally, use command line based editors like GNU Nano or Vim which require keyboard based navigation of text. Mono-spaced fonts mean the cursor moves in predictable ways when dropping a line or moving about. With a font like this that would (probably) cause problems.
It actually comes with a monospaced variant.
Indeed it does, but the main idea presented by the website is an alternative to monospaced fonts which is an interesting and novel idea but impractical in a few scenarios as mentioned before. I work mainly in Brackets and for my on screen work, a proportional font would be perfectly usable, I however stick to Source Code Pro because I like the even pacing of the code on the screen.
I couldn't quite get used to the proportional version, but I switched from Source Code Pro to Input Mono a while ago. The Extra Light weight looks beautiful on a retina screen.