Spending more time in Zone 3 is harder, sure, but it isn't more beneficial than spending that time in Zone 2. You're training different metabolic states, and working harder/suffering more doesn't actually matter.
Yes. But if we're at the level where we care about zones and total intensity because of recovery, one probably isn't doing a combined workout at all. Either it's a low intensity or a high intensity workout, not a combination of both.
Not my experience at all. Plenty of people do long runs (2h+) with faster portions. Maybe it turns out it is inefficient but it is a common practice among marathon runners.
There's a difference between a faster portion and Tabata portion. Mixing in some faster cadences helps keep things interesting even if there isn't a huge physiological benefit.
