Every decision that you make as an athlete or coach, from the perspective of triathlon training stress, is typically based on some balance between race specificity and what is physiologically best to continue progress.
Sometimes the most race-specific workouts may not be the best for long term physiological development. Conversely, sometimes these objectives align well and make the athlete's planning a bit easier.
Consider the athlete who is extremely aerobic in nature, either having been born as such or having developed that way after many years (8,000+ hours) of aerobic training.
More: Aerobic Threshold Training
Many times, this type of athlete will respond best to short, highly intense intervals. In fact, it is often these very athletes who require this type of anaerobic training in order to continue any significant long term aerobic development.
These are the athletes who, when given the option to do a 20-minute all-out effort or a three-minute all-out effort, will choose the 20-minute effort without a second thought. Typically, the maximum average pace that these athletes can sustain over only three minutes, is very similar (on a relative basis) to that which they can sustain over a full 20 minutes.
This is the same athlete whose Olympic-distance race pace is only minimally faster than their 70.3 race pace. Simply put, they lack that extra gear. Continued aerobic work would be the athletic equivalent of trying to get water from stone, so it calls for a little change of pace early in the season to get those gears back.
More: How to Create Your Own Interval Workouts
In the battle of race specificity and physiology, these athletes will typically be best served with the physiological component when 16 to 30 weeks out from race day.
As race day draws closer and closer, be it a sprint race or full Ironman, it is absolutely essential to include as much race specific training as possible (aerobic in this case). Even if early season short intensity work is required, aerobic efficiency will still be needed for any long course event.
However, due to the Ironman's very long duration, the aerobic physiology of most athletes simply does not meet its demands. As a result, this type of short repeat work should only be done after a solid period of aerobic development, and fall during the final 12 weeks before race day where it is still useful for "sharpening".
Traditionally, long course athletes have avoided high intensity intervals opting for more race specific, longer aerobic workouts. But, higher intensity intervals provide a more efficient and powerful anaerobic energy system; the ability to operate at intensities beyond anaerobic threshold. This is a tool, not necessarily required for good Ironman racing, but sometimes required to continue aerobic development over the long haul.
More: How to Maximize Your VO2 Max Training