Why? Mostly for fun. I really like it so far though, it was really simple and easy to do. I've had alot of issues with RSS where I would have to escape a ton of stuff just to get it to validate. So far with JSON Feed, I have had no issues.
It's also a very nice feed to look at. You can almost read my blog entirely by just looking at the JSON Feed of it.
There aren't alot of feed readers that support JSON Feed, yet, but here is one that does: http://json-feed-viewer.herokuapp.com and you can try viewing my feed there.
I also updated my Github repo for lambblog so you can see how I made the implementation: https://github.com/lambdan/lambblog/blob/master/jsonfeed.php
In other news, I've been busy with other stuff so I've been kind of dead on Twitter and here. That should hopefully be over soon though.