I don't really view it as a system, I view it more as a set of principles regarding how to effectuate certain adaptations.
I think the adaptations that Daniels focuses on are more helpful with respect to runners running 3000m and up rather than the 800. 1500 is sort of in between.
Here is how I view Daniels' principles:
Our ability to run fast for extended periods is limited by certain physiological factors. First, our bodies have to produce energy, and they do that by getting oxygen to the muscles engaged in the activity in question. Getting as much oxygen to those muscles as possible is the purpose of employing vo2max intervals as suggested by Daniels. For 800m runners, the ability to get enough oxygen to your muscles is not as much of a limiting factor on performance as it is for 5000m runners, so that adaptation, while important, is less important.
That oxygen is processed in the mitochondria contained within the cells, so increasing the number of mitochondria allows for greater and more efficient energy production. Running a easy distance pace is very effective at increasing the number of mitochondria. Again, since the need for oxygen at 800m is more limited as compared to the longer distance races, the need for this adaptation is lesser. That is why 800m runners don't need to runner high mileage to succeed.
When that oxygen gets to your muscles and is used in conjunction with the glucose delivered to those muscles, you produce lactic acid and waste products. The build up of those waste products signals your body that it needs to slow down. Adapting to the build up of those waste products and getting your body to clear the waste more efficiently so that it isn't signaled to slow down is achieved by training at a pace that causes slow build up of lactic acid over time - this is tempo pace. This is probably something that is pretty helpful for 800m runners.
And then there is the issue of using your energy in an efficient way, by not wasting energy on extraneous and unnecessary motion in your running form. This is achieved by running at relatively fast paces - our bodies tend to naturally get into good alignment at about mile race pace, so Daniels advocates running reps at that pace so that our bodies get used to running efficiently under stress. This would also be helpful to 800m runners.
But none of that is really all that specific to the 800m, and it does not provide the bulk of what 800m runners need. That is why I think the principles espoused by Daniels (again, I will forego calling it a system) are more useful for the longer distances.