As far as chapter 2 is concerned, one big aspect of Football is that you're still playing when the ball is out. You're still thinking, you're still re-positioning, you're still trying to outwit your marker, it's still all on. It's one of the things that makes "football fitness" unique from many other sports.
Sorry to parrot the slippery-slope argument (but let's be honest) the second the clock stops when the ball goes out, next year we'll have a timeout or something, and then next year we'll have tv ad breaks. No thank you.