Why do you meter the highlights?
When I first started taking digital photos, I was never quite sure how to expose. I went back and asked some of my past mentors and college professors, but I never quite got a satisfactory answer. While with film exposure techniques are pretty well established, digital photographers often are confused about where to meter from. As a result, the spot meter is often neglected or under-utilized. My first approach was to treat it like b/w zone system, thinking of curves as development time, but then I started learning more about the advantages of "exposure to the right of the histogram". This technique encourages the photographer to over expose as much as possible, and then compensate in post. The technique provides as much light as possible to enter the sensor such that useful information is more likely to drown out noise. As a result, I slowly started approaching digital photography more like color slide film.
Certainly I could meter at the mid-tones and expose uncompensated, or I could expose at the shadows and stop down three times. However, if I did that, the exposure might be "good", at least according to the histogram, but the data would not necessarily be. Depending on the scene, such an exposure might be a bit noisier and less flexible as a result of decreased signal to noise ratio, especially in the shadows. Overall S/NR would decrease than if I increased exposure. This is why many photographers "expose to the right".
Because I know my camera can handle hilights four and two thirds stops above meter without blowing out, placing hilights at EV +4.6 ensure that the exposure is pushed as far as possible to the right of the histogram without blowing out. Consistently, even one third stop more exposure will result in blown out hilights.
Likewise, because zones IIX through IX are placed at the outer limits of my camera's latitude, any shadows at this point that block up would be beyond my equipment's capability. I could provide more exposure to obtain greater shadow detail, but doing so the hilights would then blow out. The only solution would be to take multiple exposures and combine them.
(I'm sometimes not so great at explaining this stuff, if you have any questions, feel free to PM me)