I cannot answer the second part of your question as I am not that educated, yet, but here is a good article that I think you should read regarding your first question:
Understanding Intervals
Edit: I don't know a ton about this particular topic, but let me explain the way I understood it.
Basically, from what I've read, the issue with HIIT is that you eventually plateau. Sure it's great for increasing cardio, but eventually you hit a wall with your Vo2 max. In order to make maximize your Vo2 max, you need to train at slower speeds, with an elevated heart rate for an extended period of time to really continue to improve.
The other opinion (depending on who you ask, I believe it's true), is that training at slower speeds for a greater duration, will increase your ability to preform at faster speeds for a shorter distance.