I'm a powerlifter, so that's the perspective that I take. The systems I care about are peak strength across multi-joint compound movements, so I focus on high weight, low reps. Strongman competitions take strength endurance, they emphasize moderate weight, higher rep exercises.

For endurance athletes, systems include respiratory efficiency, VO2max, glycogen storage, among others. Some of those are best trained at low intensity, high volume. Some of those are best trained for at high intensity, low volume. None of those are best trained for at moderate intensity.

When you've reached the level of adaptation where you need to manage your effort/recovery cycle, it becomes important to avoid training efforts that don't optimize for the systems you wish to improve; there's a real recovery cost for little gain. What jedbrown is saying absolutely makes sense. It's also of lesser benefit to anyone who isn't training competitively.

-- http://fastforwardtriathlon.com/wp-content/uploads/2010/08/I...

It's oft-discussed in training plans across endurance sports. Here are a few showing up in a quick search.

http://www.cyclesportcoaching.com/articlejunkmiles.html

http://www.marathon-training-schedule.com/heart-rate-trainin...

http://coachlevi.com/training/junk-miles-what-why-avoid/

Of course if you're training for general fitness rather than to optimize performance, then zone 2 training is fine. It burns calories and you feel like you get a workout without the pain and injury potential of higher intensity training.

This is an accessible review article: http://www.fitnessforlife.org/AcuCustom/Sitename/Documents/D...

On mobile, but Google "junk miles"