When training competitively, you want to provide a stimulus to the systems you're improving. Any stimulus will require a recovery period, so ideally you only provide the stimuli that affects the systems you need to improve, to minimize recovery time and maximize improvements.
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.
While these high-intensity sessions
are believed to be critical to achieving maximal
performances, they cannot be performed optimally
if intervening basic endurance sessions are performed
at too high an intensity (Bruin et al., 1994). However,
high-intensity training sessions appear to be well
tolerated when variation in intensity of training is
ensured (Lehmann et al., 1991, 1992). Less experienced athletes may tend to train harder than prescribed during low-intensity sessions and not hard
enough during prescribed high-intensity sessions
(Foster et al., 2001).
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.
It's pretty well established in endurance training circles. Train hard or train easy, but that point just below 'hard' just exhausts you without providing training benefit.
