I don’t how to make this post a sticky but I have answered this several times before. Its a good question so here comes the loooonnng answer:
For well trained endurance athletes, which was what my background as an athlete and coach has been, Z2 IS a dangerous zone to spend too much time in. The reason is this: A well trained endurance athlete will have an aerobic threshold very close to his/her anaerobic or lactate threshold. Often these two important metabolic points will be separated by as little as 6-7% in HR. That is going to be around 10 beats. That gives a very narrow Z3 (Aerobic threshold, recall is at the top of Z2 not as the misprint in the book saying Z1). So when the fit athlete is training near AeT he/she is also training near AnT. AeT running pace will often be within 5% of AnT running pace. For these fit athletes, that’s FAST. Too fast to train a high volume. Even though the metabolic stress is the same for any athlete (fit or not) in this aerobic zone, these speeds impose a very high neuromuscular load that needs more time for recovery. Doing a high volume of Z2 for these folks will lead to overtraining. They need to do most of the aerobic base work in Z1 and then do more high intensity work at Z3 and Z4.
However, when a person with aerobic deficiency (ADS) runs at AeT (top of Z2) the pace will be quite slow. Often these folks are dismayed how slow they must run to stay in their aerobic zone. For them the neuromuscular loads are quite low at these slow speeds. So, those with ADS can handle all or nearly all of their aerobic base building work right up at their AeT. For them Z2 is NOT a black hole.
How did this make it into the book? With my rather skewed, high level athlete and coaching experience I was not fully conversant in ADS as it presents in recreational and none endurance athletes. I have learned a lot since that time and now see that in order for those with ADS to see their maximum gains they need to train right near AeT.
I hope this helps