I first heard something akin to Zulu's theory about 10 years ago in a talk from Dave Scott (the triathlete). He was talking to a group of triathletes and encouraging everyone to work out with a HRM, which at the time was fairly new. I went home and bought one and have used one pretty faithfully since. Like Zulu, it gives my ADHD brain something to do to distract from the boredom of running.
I'm a doctor with a special interest in sports medicine (not fellowship-trained, however) and have tried to keep up on the research regarding HRM training. On the other hand, I'm pretty slow, due to a combination of lack of time to train and genetics. I'm more of a teacher than a doer, so take this for what it's worth.
I did try the Zulu approach where I tried to keep all my runs within my aerobic zone. End result: I was much slower at all distances. Which parallels both common sense and the consensus on HRM training.
Most runners will do four different types of workouts: Track/Speed, Tempo, Long, and recovery. To figure out your optimal HR, you need to figure out which type of run you're doing. An athlete's maximum HR is variable depending on the person, and slowly declines as you age. You can't influence it with training. Your anaerobic threshold is typically about 85% of your max HR, and signifies the point at which your muscles can no longer undergo aerobic metabolism (the most efficient, but requires oxygen) and swith to anaerobic metabolism (less efficient, doesn't require oxygen, produces lactic acid as a byproduct). You CAN influence this anaerobic threshold with training; well-trained athletes can go up to 90-95% of their max HR without going anaerobic. For most people 85% of estimated max HR is a good guess; if you want an exact number, you have to go into a lab to get tested. The other feature of a well-trained athlete is that they can tolerate much higher levels of lactic acid in their muscles before failure; typical person can tolerate lactic acid levels of 1-2 while an athlete can go as high as 6-7. So, a well-trained athlete can both go at a higher HR before going anaerobic, and can go anaerobic for much longer times before muscle failure.
Your stroke volume (amount of blood pumped with each beat of your heart) can be influenced by aerobic training, which will allow you to reach moderately higher levels of exertion without your heart rate increasing. However, the lactate threshold and anaerobic threshold, both very important to performance, cannot be influenced without exiting your aerobic zone. Additionally, the neuromuscular training gained particularly from track/speed workouts, and one of the most important aspects of speed, will be ignored with solely aerobic workouts.
So, back to the four workouts:
1) Speed/Track - Your point is to increase your lactate and anaerobic threshold and improve your neuromuscular training ("step turnover"). For these, you can go 90-95% of your max HR.
2) Tempo - You're combining aerobic efficiency with some effect on anaerobic threshold. You should not exceed an hour, but probably should be in the 80-90% range with a brief interval up to 95%.
3) Long - Your point is solely to get your muscles used to the pounding and motion they'll receive. You should NEVER go above 85% on these. Remember, that once you're going anaerobic, you're building up waste byproducts and the muscle cells are not being adequately nourished. You're risking injury by combining length and speed.
4) Recovery - The point is active recovery, i.e. less than an hour, never anaerobic.
To answer creekster's question about "bonking". Your muscles always burn their glycogen stores first, regardless of your speed. These are gone in 30-45 minutes; if you want to burn fat while exercising, you need to exceed this time. You'll be able to go much longer (and burn more fat) if you stay aerobic.
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