Hunter and I did a podcast with FastTalk* a few years ago, the title of which was something like, “Do we even need training zones?”. I think that they were hoping to stir up a bit of controversy.
If so, I think that they may have been somewhat disappointed, because my position was (and is) that the primary purpose of such (rather arbitrary) distinctions between exercise intensities is simply to aid in communication.
Example: When I go for a ride and decide to keep it easy, I know what that feels like. However, when a coach tells someone else to go for a ride and keep it easy, how do they best convey that idea to that individual? Having a cogent way of classifying training intensities aids in that process.
As you alluded, where things go off the tracks is when people take such “zones” too literally, mistakenly believing that you can very specifically target particular training adaptations by getting the exercise intensity “just right”. (Even worse is thinking that you can do so with particular workouts, e.g., the mistaken faith of coaches and athletes in “over unders” as a way of specifically enhancing lactate clearance.)
At this point an astute reader might be adding 2+2 and interpreting my original post and my comments above as endorsing the “Norwegian 3 zone” system. This would be incorrect, however, as
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the three types of workouts I originally described don’t align with that approach (which crudely lumps everything above maximal metabolic steady state together… maybe fine if you’re a distance runner or triathlete, but even road cyclists have to frequently sprint, and then there are the trackies and BMXers to think about); and
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IMO, having only 3 zones to cover everything below maximal metabolic steady state is too crude of an approach.
Combining 1 and 2, I decided that the optimal number of levels (not zones) is 7 - enough to be sufficiently precise, but not one more than necessary. (Then along came the fans of “sweetspot” training like Hunter and Frank Overton, clamoring for it to have a special place in the system, followed by Tim Cusick’s desire to further capitalize on the WKO4 P-D model, and things have gotten messy again. But IMO, 7 is still the perfect number.)
*Never again - I don’t like the way that they reuse clips to weave together subsequent episodes, making it seem as if you were interviewed about that new question and in the process indirectly putting words in your mouth supporting their slant. No professional journalist would ever do that, nor would their editor let them if they tried.
ETA: Of course, clearly a contributing factor to this trend was Polar’s successful commercialization of wireless HR monitors. Before that, there wasn’t much need for “zones” in “free range” sports such as cycling, as you couldn’t precisely control the training intensity anyway. It was challenging even in running, because although pace is a perfectly valid metric, not every course is marked, or sufficiently flat. Coaches could/would of course prescribe very precise workouts on the track, but unless you were Emil Zapotek (or a swimmer), such training was the icing on the cake, not the cake itself.