I'll chip in, because I've dropped 50lbs in a year, and the cornerstone of my cardio work was fasted swimming and running. Just to add I didn't take in any food after 6pm, all my eating was done before that.
Main reason I went fasted was, to be honest, because I couldn't be arsed to go and do HIIT once I'd worked out in the afternoon.
Depends on whether you can get up and hit the road at 5 or 6am six days a week. I'm an early bird, so you see again that played into the equation.
The watchword behing the theory as well is that because you're in a fasted state there should be very little glycogen for your body to burn thus forcing your body to burn fats and small amount of muscle tissues to fuel itself. I mainly saw benefit in that when I ate breakfast (45-60 minutes) after finishing it felt like my metabolism was running at a million miles an hour.
One of the theories bandied around about HIIT is that it "tricks" your body into skipping past its glycogen stores and burning fat.
I'll leave the science for someone else. I personally think people respond to different things with cardio similar to how people react to different weight training regimes.